GB2230155A

GB2230155A - Voltage-controlled current source

Info

Publication number: GB2230155A
Application number: GB8907303A
Authority: GB
Inventors: Rodney James Lawton
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1989-03-31
Filing date: 1989-03-31
Publication date: 1990-10-10
Also published as: WO1990012454A1; GB8907303D0

Abstract

A voltage controlled current source for a gyrator circuit, the current source comprising: a differential amplifier (30) including first and second transistors (32, 34) whose bases form the input ports of the current source and whose emitters are connected in common in a long-tailed pair configuration to a current source (40); and the collector loads of the transistors each comprising a resistive means (40, 42) and a negative resistive means, the negative resistive means being controlled by the collector voltages such that the value of the effective negative resistance combined with the value of said resistive means provides an effective infinite output impedance and compensates for losses arising within the collector load and associated transistor.

Description

GYRATOR CIRCUITS The present invention relates to gyrator circuits.

Gyrator circuits are well described in the literature, see for example ESSCIRC '85: tilth European Solid State Circuits Conference, September '85, Toulouse, France; "Analog Integrated Filters or Continuous Time Filters for LSI and VLSI"; pp. 292 to 292c, J.O. Voorman.

Referring to figure 1 of the accompanying drawings this shows the basic gyrator function wherein the gyrator circuit 2 is represented as a four port device with the output ports 4 being coupled to a load impedance Z such that the input impedance at the input ports 6, Zin. is as follows: Zin = G2/Z, where G is an impedance factor of the circuit.

A common implementation of the gyrator function is shown in the circuit of figure 2 wherein one of the input ports and one of the output ports are grounded so as to provide a single input port 6 and a single output port 4. Two voltage controlled current sources 8, 10 (VCCS) are provided having, ideally, an infinite input impedance and an infinite output impedance.Current source 8, connected in feed forward configuration between the input and output ports has a positive mutual conductance gm such that Iout = Vjngm. Current source 10 has a negative mutual conductance (-gm) such that 1out = gmVi,. In this case: Zm = (Zg2m)-1 A typical circuit for a voltage controlled current source of a gyrator is shown in figure 3 as comprising a differential amplifier comprising transistors 20, 22 connected in long-tailed pair configuration with a current source 24 in the tail, outputs being taken from the collector loads 26, 28.A modification of this basic differential amplifier is shown in figure 4 wherein the collector loads are replaced by transistors 30, 32, the bases of the transistors being tied in common to a fixed bias potential. The use of a differential amplifier has the advantage that it has a very high input impedance and high output impedance. The added advantage of the circuit in figure 4 is that the transistor loads 30, 32 can be represented as current sources with very high impedance, and hence the varying output current provided by the differential amplifier is not dissipated in the collector load nor affected by the load impedance coupled to the output ports. Hence the operation of the gyrator circuit may be more accurately controlled.

However, for very high frequencies of operation such as may occur in a radio receiver where signals of the order of tens or hundreds of megahertz may be encountered, the basic differential amplifier configurations shown in figures 3 and 4 do not provide a perfect voltage controlled current source. In particular, for the circuit shown in Figure 3, the relatively low output impedance contributes to a low Q factor (where Q = Zout/gm)- 100. For the circuit shown in Figure 4, whilst Q is high for low frequencies (=103) since transistors 30, 32 are of n-p-n type, they exhibit high parasitic capacitance at high frequencies which drastically limits their performance.

Various construction of gyrator circuits have been proposed to deal with specific defects in the practical form of an ideal voltage controlled current source, see in particular pp 292 b,c of the above quoted paper. However as appears from Figures 17, 18 of the paper, none of these proposals has addressed the problem of dealing with high frequency operation of a gyrator circuit and maintaining a high Q factor.

With a view to meeting this problem, the present invention provides a voltage controlled current source for a gyrator circuit, the current source comprising:a differential amplifier including first and second transistors whose bases form the input ports of the current source and whose emitters are connected in common in a long-tailed pair configuration to a current source; and the collector loads of the transistors each comprising a resistive means and a negative resistive means, the negative resistive means being controlled by the collector voltages such that the value of the effective negative resistance combined with the value of said resistive means provides an effective infinite output impedance and compensates for losses arising within the collector load and associated transistor.

In a preferred embodiment, the negative resistance means is provided by a further differential amplifier comprising third and fourth transistors wherein the base of the third transistor is coupled to the collector of the first transistor, the base of the fourth transistor is coupled to the collector of the second transistor, the emitters of the third and fourth transistors are coupled in long-tailed pair configuration, the collector of the third transistor is connected to the collector load of the second transistor, and the collector of the fourth transistor is connected to the collector load of the first transistor.

Thus with this arrangement when current flow increases through the collector path of the first transistor, some of this current flow being dissipated within the collector load and first transistor thereby causing a change in output impedance, there is created an increased current in the collector path of the associated fourth transistor which compensates for the losses in the collector path of the first transistor.

The third and fourth transistors have their emitters coupled to a constant current source via respective third and fourth resistive emitter loads, the emitter loads serving to control the amount of current supplied to the first and second transistors. If the emitter loads were not provided, the third and fourth transistors would form a latch circuit and would not operate correctly. It can be shown that for optimum operation of the circuit according to the invention the value of an emitter load resistor should be approximately equal to but greater than the value of the collector load resistor of the respective first and second transistor. Since all transistors may be of the n-p-n type, large values of parasitic capacitance may be avoided at high frequencies even in the GHz region.In addition the Q factor of the circuit in accordance with the invention is high since the effective output impedance is practically infinite. The Q factor is limited in practice by the tolerance of circuit components, and amounts to the Q of the basic differential amplifier multiplied by a factor of about 100.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figures 1, 2, 3 and 4 are circuit diagrams included for the sake of explaining the operation of the gyrator and showing known voltage controlled current sources for use in a gyrator; and, Figure 5 is a circuit diagram of a preferred embodiment according to the invention.

Referring now to figure 5 there is shown a voltage controlled current source in accordance with the invention having an effective infinite input impedance and effective infinite output impedance at all frequencies suitable for HF radio receivers, i.e. up to -100MHz.

The circuit comprises a differential amplifier 30 comprising first and second NPN transistors 32, 34 whose bases form the input ports to the circuit 36, 38 and whose emitters are connected in longtailed pair configuration to a common load formed by a constant current source 40. The collectors of the transistors 34, 36 are connected to collector loads 40, 42 of equal value R1 and to output ports 60, 62. The collectors of transistors 32, 34 are also connected to a further differential amplifier 44 comprising a third NPN transistor 46 and fourth NPN transistor 48, wherein the base of transistor 46 is coupled to the collector of transistor 32, the base of transistor 48 is coupled to the collector of transistor 34, the collector of transistor 46 is coupled to the collector of transistor 34, and the collector of transistor 48 is coupled to the collector of transistor 32.

The emitters of transistors 46, 48 have emitter loads R2 50, 52 and are connected in long-tailed pair configuration to a constant current source 54.

Thus in operation, for a high frequency (hf) input voltage at the input ports 36, 38, consider the case where the voltage of the base of first transistor 32 increases thereby increasing the current through resistor 40. Since the resistor has a finite value R1, and since losses will be associated with capacitor 32 by virtue of parasitic capacitances etc. the voltage at output port 60 would vary, and the circuit would deviate from the ideal of an infinite output impedance if differential amplifier 44 were not provided. However at the same time that current increases in transistor 32, the voltage at the base of transistor 34 will decrease resulting in an increased voltage at the base of transistor 46 with an associated increased current flow through transistor 46 and resistance R2.

This increased current flow, sourced from current source 48 will be controlled by the associated feedback voltage across emitter load resistor 52. The value of R2 will be chosen such that the amount of current supplied to the collector path of transistor 32 will compensate for that dissipated in the resistive load 40 and parasitic capacitance of transistor 32. Thus differential amplifier 44 acts as a negative impedance and the circuit behaves as if there were no losses nor finite impedance effects in amplifier 30.

Expressed mathematically, if an increase of base voltage of transistor 32 causes an increase of collector current dlc, there will tend to be a change in output port 60 voltage of dICRl. However there will be be a corresponding increase in collector current in transistor 48 of dlc creating a voltage dIcR2 across resistor 52. This will be reflected through transistor 48 at output port 60, and as a rough and ready measure: dICR2 ~ dIcRl; hence R1 = R2 Thus effectively the transistor 48 and emitter load 52 provide a negative resistance for a "composite" collector load of transistor 32, in that increased current flow through the collector of transistor 32 tends to raise the collector voltage. For correct operation, R1 > R2; otherwise a bistable latching effect may be created.

Whilsts in the specific embodiment described above, the negative impedance is provided by a differential amplifier having emitter load resistors, any other known type of negative impedance circuit may be provided, e.g. an active filter circuit, a configuration of op. amps.

Claims

1. A voltage controlled current source for a gyrator circuit, the current source comprising: a differential amplifier including first and second transistors whose bases form the input ports of the current source and whose emitters are connected in common in a long-tailed pair configuration to a current source; and the collector loads of the transistors each comprising a resistive means and a negative resistive means, the negative resistive means being controlled by the collector voltages such that the value of the effective negative resistance combined with the value of said resistive means provides an effective infinite output impedance and compensates for losses arising within the collector load and associated transistor.

2. A source as claimed in claim 1, wherein the negative resistance means is provided by a further differential amplifier comprising third and fourth transistors wherein the base of the third transistor is coupled to the collector of the first transistor, the base of the fourth transistor is coupled to the collector of the second transistor, the emitters of the third and fourth transistors are coupled in longtailed pair configuration, the collector of the third transistor is connected to the collector load of the second transistor, and the collector of the fourth transistor is connected to the collector load of the first transistor.

3. A source as claimed in claim 2, wherein the third and fourth transistors have their emitters coupled to a constant current source via respective third and fourth resistive emitter loads, the emitter loads serving to control the amount of current supplied to the first and second transistors.

4. A voltage controlled current source for a gyrator circuit substantially as described with reference to the accompanying drawings.