GB2281160A

GB2281160A - Compensation for temperature drift and input offset in differential amplifiers

Info

Publication number: GB2281160A
Application number: GB9416547A
Authority: GB
Inventors: Peter Fred Blomley
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-08-17
Filing date: 1994-08-16
Publication date: 1995-02-22
Anticipated expiration: 2014-08-16
Also published as: GB9317075D0; GB2281160B; GB9416547D0

Abstract

The operation of a signal processing apparatus, such as an amplifier or an analogue to digital signal converter (ADC), is enhanced by connecting a preliminary amplifier 12 upstream of an amplification stage 1 of the apparatus so as to receive the same input signal as a first input of the amplification stage and to provide, an output signal which is combined with a feedback signal derived via at least a resistive path from the output of the amplification stage (either directly or, for an ADC through a clocked flip-flop and an integrating capacitor), the combined signal being fed to a second input of the amplifier stage. The arrangement compensates errors arising from eg. temperature drift, input offset voltage or current. <IMAGE>

Description

Title: Improvements in and relating to Signal Processing ApParatus Field of the invention This invention relates to electrical signal processing in particular to signal amplification using apparatus based on so called operational amplifiers, to a method of supplying signals to and deriving signals from an amplifier so as to enhance the operation thereof and to an accessory for an operational amplifier.

The invention is also applicable to other signal processing apparatus, for example certain analogue to digital signal converters and the like, which include a feedback loop.

Background to the invention There are various ways in which an operational amplifier can be made to function so as to compensate for errors, arising from, for example, temperature drift, of input offset voltage and current which are present in the amplifier output. For example, an operational amplifier can be connected in a feedback loop so that its output signal (or a proportion thereof) is fed back to one of the amplifier inputs, usually an inverting input.

Additionally or alternatively, the operational amplifier can be connected to a second amplifier, usually called an error amplifier, which generates an error compensation signal which is combined with the output of the operational amplifier.

The open loop gain of the operational amplifier would have to be infinite if either of these methods were to result in the complete elimination of errors. Since this is not possible, in practice, some errors will still appear at the output of an operational amplifier.

Summarv of the invention According to a first aspect of the invention a method of enhancing the operation of an amplifier, having first and second inputs and an output, comprises the step of connecting a preliminary amplifier upstream of the second input of the amplifier so as to receive the same input signal as the first input of the amplifier and provide, in combination with a feedback signal derived via a resistive path from the amplifier output, an input signal for the second input of the amplifier.

Preferably the amplifier is an operational amplifier.

Thus in a circuit including an operational amplifier having first and second inputs, an input signal to the first input of the operational amplifier is also fed to the preliminary amplifier, the output of which is combined with the feedback signal derived from the output of the operational amplifier to form a second input signal which is fed to the second input of the operational amplifier.

The preliminary amplifier is found to substantially eliminate errors at the inputs of the operational amplifier and hence at the output thereof, irrespective of the open loop gain of the operational amplifier.

Preferably, the output signal of the preliminary amplifier and the said feedback signal pass to ground through a resistive path connected in parallel with the output of the preliminary amplifier and the resistive path by which the feedback signal is derived.

Preferably, the second input signal and the feedback signal are fed to the inverting input, and the first input signal is fed to the non-inverting input of the operational amplifier.

The feedback signal may also be fed to a further input of the preliminary amplifier.

According to a second aspect of the invention, there is provided signal processing apparatus including an amplifier stage having a first input for connection to a source of signal which is to be processed, a second input, and an output, the apparatus further comprising feedback means for relaying a negative feedback signal, derived from the output of the amplification stage, to said second input and a preliminary signal amplifier having an input connected to the source in parallel with said first input and the output connected said second input in parallel with the feedback means.

The apparatus may be so arranged as to function as a signal amplifier.

Preferably the amplifier stage comprises an operational amplifier, and the first input is a non inverting input and the second input is an inverting input thereof.

Where the apparatus is to be used as a current amplifier, the output of the preliminary amplifier may to advantage also be connected to ground in parallel with the feedback circuitry through a further resistance.

The preliminary amplifier may be a line amplifier or a differential amplifier. In the latter case the input signal is preferably fed to the inverting input of the differential amplifier, the non inverting input of the differential amplifier being connected to the output thereof.

Where the apparatus comprises a voltage amplifier, the output of the preliminary amplifier is preferably connected to the feedback circuitry (and hence the second input of the amplifier stage) through a further resistive path. In this case, the preliminary amplifier preferably also comprises a differential amplifier having a first input connected in parallel with the first input of the operational amplifier and a second input connected to the feedback circuitry.

Alternatively, the apparatus may comprise an analogue to digital signal converter, the amplification stage comprising a differential amplifier, the apparatus further comprising conversion means connected to the output of the differential amplifier, and operable to generate a pulse train when the magnitude of the output from the differential amplifier exceeds a threshold, wherein the feedback circuit means derives a negative feedback signal representative of a parameter of the pulse train so that said parameter is representative of the magnitude of input signal fed to the amplifier stage.

The invention also lies in an accessory for an amplifier, the accessory comprising a preliminary amplifier having an input adapted to be connected in parallel with a first input of the operational amplfier, and an output adapted to be connected to a second input of the operational amplifier in parallel with a feedback loop connecting the output of the operational amplifier to its second input.

Since the preliminary amplifier is situated upstream of the amplifier inputs, the invention provides a relatively straightforward way of upgrading the performance of an operational amplfier by retro-fitting a device as aforesaid.

Brief description of the drawings The invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram showing a conventional arrangement of an operational amplifier with negative feedback; Figure 2 is a block circuit diagram showing current amplification apparatus in accordance with the present invention; Figure 3 is a block circuit diagram showing voltage amplification apparatus in accordance with the present invention; Figures 4 and 5 show various possible modifications of the apparatus shown in Figures 2 and 3; Figure 6 is a circuit diagram of amplification apparatus in accordance with the invention; Figures 7-11 are graphs illustrating various aspects of the performance of the conventional arrangement shown in Figure 1 and the apparatus shown in Figure 2; ; Figure 12 is a block circuit diagram of a digital to analogue signal converter in accordance with the invention; Figure 13 shows apparatus for testing the effect of a preliminary amplifier; and Figures 14 and 15 are graphs illustrating various aspects of the performance of the apparatus shown in Figure 13.

Detailed Description In the conventional apparatus shown in Figure 1, an operational amplifier 1 has a non inverting input 2 connected to a source 4 of input signal, and an inverting input 6 connected to earth through a resistance 8 of Rl ohms. The output of the operational amplifier 1 is also connected to the inverting input 6 in parallel with the resistance 8 through a resistance 10 so as to provide the feedback loop for the amplifier.

A simplified calculation of the output voltage (rout) is set out below Where # V represents the voltage difference between the inputs of the amplifier 2, the voltage across the resistance 8 can be represented as Vin - # V Where Vin is the voltage on the input signal.

For the purposes of these calculations it is assumed that the input resistance of the amplifier 1 is infinite, as a result of which the current (I) flowing through the resistance 8 is the same as that flowing through the resistance 10. Hence the total voltage drop across the resistances and hence the output voltage Vut is given by Vout = I (R1 + R2) By virtue of ohms law, Vin - A V I = R1 Therefore Vout = R1 + R2 [Vin - # V] R1 and since L V = Vout G (Where G is the open loop gain of the amplifier 1)

The second term in the brackets of the final line of the above calculation can be considered to be the term which gives rise to errors in the amplifier output (arising from variations in G).

Thus that only approaches zero as G approaches infinity.

Apparatus in accordance with the invention can be made by modifiying the Apparatus shown in Figure 1 in the way shown in Figure 2 ie by the addition of a preliminary amplifier 12. The preliminary amplifier 12 is a differential amplifier, the inverting input of which is connected to the source 4 in parallel with the input 2 of the operational amplifier. The non inverting input of the amplifier 12 is connected directly to the amplifier output.

The output of the amplifier 12 is also connected to the input 6 and the resistor 8 in parallel with the resistor 10.

With this arrangement, the current, Ic, fed by the preliminary amplifier 12 through the resistor 8 modifies the mathematical representation of the voltage across that resistor to: Vin - A V + ICR Thus, in this case, the output voltage, Volt, is given by the equation Vout = R1 + R2 [Vj - A V + Ic R] Ri since

= R1 + R1 [Vin] R1 In this equation Vout is therefore independent of G and hence of any errors arising from flucuations in the magnitude of G.

Thus according to this simplified calculation, the preliminary amplifier serves to prevent any errors in the amplifier 1 from having any effect on Volt, and in practice serves substantially to reduce said errors.

Figure 3 shows voltage amplfication apparatus in which an operational amplifier 14 has non inverting and inverting inputs 16 and 18 respectively connected to a source of input signal 20 and to the amplifier output through a resistance 22. In this arrangement, a preliminary amplifier 24 comprises a differential amplifier having an input 26 which is also connected to the source 20, and another input 28 connected to the output of the amplifier 14 through the resistance 22 and, in parallel, to its own output through another resistance 30.

As with circuit shown in Figure 2, the output of the preliminary amplifier 24 counteracts the error in the input voltage for the amplifier 14.

The apparatus shown in Figure 4 is similar in many respects to that shown in Figure 3, and accordingly corresponding components are indicated by like reference numbers, raised by 100. In this case, the output of the preliminary amplifier 124 is connected to the feedback loop for the amplifier 114 at a position between two series connected resistors 150 and 152 in the loop.

In the apparatus shown in Figure 5, components corresponding to those of the apparatus shown in Figure 3 are indicated by like references raised by 200.

In this case a signal from a source 220 is fed to the noninverting input 216 of an operational amplifier 214. The input signal from the source 220 is also fed to the inverting output 226 of a preliminary amplifier 224, the output of which is connected to the inverting input of the amplifier 214 in parallel with the resistor 220. The output of the amplifier 224 is also connected to the non inverting input 228 of the latter.

It will be seen that the resistor 220 is connected to the output of the amplifier 214 through two further, series connected amplifiers 260 and 262, and that the output of the amplifier 216 is connected to the inverting input 218 of the amplifier 214 through a capacitor 216, and to its own input through a capacitor 266.

Figure 7 is a graph showing a computer simulation of the output of the amplifier shown in Figure 1, where Rl is set at 1 kilo-ohm and R2 is set at 99 kilo-ohms. Line 1 represents a sinusoidal input signal, which gives rise to the output shown at line 2.

As can be seen, the output signal differs noticeably in both phase and magnitude from the input signal.

By contrast, referring to Figure 8, the same simulation for the circuit shown in Figure 2 gives rise to an output signal with very little difference from the input signals.

Figure 9 illustrates the improvement of gain and phase difference which can be achieved with the apparatus of Figure 2 over that of Figure 1. Figure 10 shows in more detail the effect of frequency on the gain of the apparatus of Figure 1, from which it can be seen that the gain starts to decrease when the frequency of the input signal reaches approximately 150 Kilo Hertz. Figure 11 shows the result of the same simulation for the apparatus shown in Figure 2. As can be seen, the input frequency reaches 3 Mega Hertz before the gain of the amplifier is affected.

With reference to Figure 13, a preliminary amplifier can also be used to reduce output errors arising from internal digital and analogue noise and errors inherent in the converter.

The converter comprises a high gain differential amplifier 300 the non-inverting input of which accepts an analogue voltage signal, Vin, to be converted.

The output of the amplifier 300 is connected to a D-type flip flop 302 which also receives a pulsed clock signal from a system clock (not shown).

When the output of the amplifier 300 exceeds the threshold voltage of the flip flop 302, the latter produces a stream of pulses of constant amplitude and duration at its output 304 at the frequency of accurence of the pulses of the clock signal.

The output of the flip-flop 302 is connected to the inverting input of the amplifier 300 via feedback circuitry which includes a resistor 306 and parallel connected capacitor 308 and resistor 310 both of which are connected to earth.

The capacitor 310 integrates the pulses from the flip flop 302 to form the main (low frequency) component of the feedback signal.

In this example the output pulses are positive and the current flowing in R2 causes the average voltage across the capacitor rise. The rise in voltage will reduce the difference between Vin and the feedback voltage. Eventually this causes the output of the amplifier 300 to fall below the required threshold voltage of the flip flop 302.

Once this happens the supply of pulses at the output 304 ceases, hence stopping any further rise in the feedback voltage. Within a few clock periods the feedback voltage will return back to normal and a limit cycle will occur which brings the error voltage (ie the difference between the voltages applied to the inputs of amplifier 300) to a minimum for that applied input voltage (van) and the type of circuit.

Thus the duration of the pulse stream (and hence the number of pulses) generated by the flip flop 302 before the output of the amplifier 300 falls below the is related to the magnitude. The pulse train from the flip flop 302 can, if required, be converted into a multi bit digital output by further digital processing circuitry (not shown).

However as in any normal feedback system there are always errors and noise created by the non-ideal nature of the principal elements of the A/D converter. These errors are caused by finite threshold voltages limited gain in the amplifier and delays in the digital circuits. Also the circuit produces quantization noise and errors caused by the finite timing nature of the pulse train.

In order to mitigate the effect of such errors the converter includes a preliminary amplifier 312 having one input for accepting the input signal Vin in parallel with the non inverting input of the amplifier 300, and another input connected to the output of the preliminary amplifier 12.

The output of the preliminary amplifier 302 is also connected to the feedback circuitry for amplifier 300 so that the output of the preliminary amplifier 300 is combined with the feedback signal derived from the pulses produced by the flip flop 302.

As before, any voltage error (or noise) which appears between the input and inverting terminals of the amplifier 300 is fed into the feedback circuits such that the total effective signal at the output (now in digital format) of the flip flop 302 is corrected so that the both transfer of the analogue signal to the digital stream format is very linear and has most of the excess (digital) noise eliminated.

The mathematical analysis of the analogue to digital converter is equivalent at low frequencies (within the bandwidth) to that of an operational amplifier with a gain dependent on the signal level. The main difference between the circuits is the nonlinear nature of the digital circuit inside the feedback loop also causes additional quanitization noise which the preliminary amplifier 312 effectively reduces in the system pass band.

The analysis of the "out of band" performance of these circuits is extremely complex and of limited use in most systems.

Figure 6 shows the trial circuit for an investigation of the "Forward Correction" technique employed in the apparatus as previously described.

The "standard" operational amplifier block (B) comprises a combination of input transistor pairs 400, 402 and a gain block 404 plus various internal load and compensation components as shown.

The inverting input to the Operational Amplifier circuit is the base of transistor 400 and the non inverting input is the base of transistor 402.

The load (internal) for this input differential pair of transistors is a "current mirror" circuit comprising transistors 406 and 408 and resistors 410 and 412. The output from this input circuit is connected to resistors 414 and 416 and capacitors 418, 420 and to the input of the gain block 404 (which has a voltage gain of 100).

The output from the gain block 404 is the actual output from the "equivalent" operational amplifier block (B).

The feedback resistors 422, 424 are equivalent to the ones shown in Figure 1 ie R7 R2 and Rs m R1. The capacitor 416 (1.5pF) is the phase compensator for the FEEDBACK loop (lead).

The "Forward Correction" circuit is shown as Block A and comprises transistors 426, 428, 430, 432, 434 and 436. This circuit block has a defined transconductance (gm) which is fixed by the value of the resistor 438 connected to the emitters of transistors 426 and 432.

The current gain of the circuit (internally) is x2 hence the value of this resistor from ideal forward connection of the input error, has to be twice (x2 R8). In this example a 1950Q resistor was used. This was slightly less than the ideal (2000Q) [ie 2 x 1000Q R8)] because of the extra 50Q resistance of the emitters of transisters 400 and 436 which is added to the total.

The input signal is fed to the bases of transistors 402 and 432 to this defined Transconductance Circuit (A) and the output from the collectors of 430 and 436 is connected to the feedback resistors (R8 = R1) hence feeding the error current into the feedback path exactly as shown in Figure 2.

The resistors 438 and 440 are the current mirror tail resistors and 442 is the beta compensation resistor for transistor 434.

Table 1 illustrates the improvement in performance of the circuit of Figure 6 as compared with an operational amplifier such as is shown in block B connected to a negative feedback loop, but not to a preliminary amplifier A. As can be seen, for 1 and 10 KH, input signals (of 0.1 volts rms) the distortion at the second and third harmonics of the circuitry shown in Figure 6 is significantly lower than that for the arrangement which lacks a preliminary amplifier.

TABLE 1 New Compenstation FB Circuit DISTORTION RESULTS:- VIN = 0.1 vrms without 1KHZ HD2 5pV (-105DBM) preliminary HD3 0.2pV (-133DBM) Amplifier 10KHZ HD2 7pV (-102DBM) HD3 0.28pV (-131DBM) with 1KHZ HD2 0.09pV (-140DBM) preliminary HD3 0.001pV (-177DBM) Amplifier 10KHZ HD2 0.6MV (-161DBM) HD3 0.009MV (-161DBM) In the apparatus shown in Figure 13, a preliminary amplifier 500 can be switched on or off by a switch 502, and the feedback loop from the output of an operational amplified 504 includes parallel connected diodes 506, 508 and a resistor 510 for creating strong distortion in the system.

The output from the apparatus is fed into a 10 ohm load 512 and, as with the arrangement shown in Figure 2, the output from the amplifier 500 is fed to the inverting input of the amplifier 504 in combination with the feedback signal.

By operating the switch 502, the effect of the preliminary amplifier on the distortion introduced by the components 506, 508 and 510 can be determined.

Figure 14 shows the relationship between input current IIN fed to those components and a output current lout passed by those components.

Figure 15 shows at trace 550, the output (at the load 512) for a sinusoidal voltage input signal when the amplifier 500 is not active. The trace 552 illustrates, for the same input, the output produced by the apparatus. It can be seen, therefore, that the amplifier 500 significantly reduces distortion in the output.

Claims

1. A method of enhancing the operation of an amplifier having first and second inputs and an output, the method comprising the step of connecting a preliminary amplifier upstream of the second input of the amplifier so as to received the same input signal as the first input of the amplifier, and provide, in combination with a feedback signal derived via a resistive path from the amplifier output, an input signal for the second input of the amplifier.

2. A method according to claim 1 in which the amplifier is a differential amplifier.

3. A method according to claim 1 or claim 2 in which the output signal of the preliminary amplifier and the said feedback signal pass to ground though a resistive path connected in parallel with the output of the preliminary amplifier and the resistive path by which the feedback signal is derived.

4. A method according to claim 2 or claim 3, in which the first input of the amplifier is its non-inverting input and the second input is the inverting input of the operational amplifier.

5. A method according to any of the preceding claims in which the feedback signal is also fed to a further input of the preliminary amplifier.

6. Signal processing apparatus including an amplifier stage having a first input for connection to a source of signal which is to be processed a second input, and an output; the apparatus further comprising feedback means for relaying a negative feedback signal, derived from the output of the amplification stage to said second input and a preliminary signal amplifier having an input connected to the source in parallel with said first input and an output connected to said second input in parallel with the feedback means.

7. Apparatus according to claim 6 in which the apparatus is so arranged as to function as a signal amplifier.

8. Apparatus according to claim 6 or claim 7 in which the amplifier stage comprises a differential amplifier the first input being a non inverting input and the second input being an inverting input, thereof.

9. Apparatus according to claim 7 in which the output of the preliminary amplifier is also connected to ground in parallel with the feedback means through a further resistance, the apparatus functioning as a current amplifier.

10. Apparatus according to any of claims 6 to 9 in which the preliminary amplifier comprises differential amplifier, the input signal being fed to the inverting input thereof.

11. Apparatus according to claim 7 in which the output of the preliminary amplifier is connected to the feedback means through a further resistive path, the apparatus functioning as a voltage amplifier.

12. Apparatus according to claim 11 in which the preliminary amplifier has a first input connected in parallel with the first input of the operational amplifier and a second input connected to the feedback circuitry.

13. Apparatus according to claim 6 in which the apparatus functions as an analogue to digital signal converter, the amplification stage comprising a differential amplifier, the apparatus further comprising conversion means connected to the output of the differential amplifier, and operable to generate a pulse train when the magnitude of the output from the differential amplifier exceeds a threshold, wherein the feedback means derives a negative feedback signal representative of a parameter of the pulse train so that said parameter is representative of the magnitude of input signa fed to the amplifier stage.

14. An accessory for signal processing apparatus, the accessory comprising a preliminary amplifier having an input adapted to be connected in parallel with a first input of an amplifier stage of the apparatus, and an output adapted to be connected to a second input of the amplifier stage in parallel with a feedback loop connecting the output of the amplifier stage to its second input.

15. A method substantially as described herein with reference to Figures 2-12 of the accompanying drawings.

16. Apparatus substantially as described herein, with reference to, and as illustrated in, any of Figures 2-12 of the accompanying drawings.