IE43640B1 - Telephone microphone - Google Patents

Abstract

1525964 Moving coil microphone circuits STANDARD TELEPHONES & CABLES Ltd 20 Nov 1975 47768/75 Heading H4J A microphone assembly adapted to be fitted to a telephone subscriber's instrument includes a moving coil transducer and a two-stage D.C. coupled amplifier for matching the transducer impedance to line. The microphone assembly is preferably arranged to have an electrical output similar to that of a carbon microphone. The transducer (11), Fig. 1, not shown, together with the amplifier (12), is mounted on a circuit board (13) and fits into a housing of a conventional telephone transmitter. The amplifier (Fig. 2) includes transistors V 1 , V 2 , V 3 forming a two-stage D.C. coupled amplifier with full D.C. and partial A.C. feedback. Transistor V 1 acts as a common emitter and amplified signals appear across collector resistor R 3 . The signal then passes to the base of compound common emitter V 2 , V 3 , the resulting amplified output appearing across R 5 and R 6 and the load at terminals T 1 , T 2 . Overall D.C. feedback is provided by R, and R 2 . The A.C. feedback provided by resistors R 1 and R 2 is decoupled by capacitor C 1 in the frequency band 300 Hz to 3À4 kHz. The only local emitter A.C. feedback is operating for the first stage gain stabilization. The gain of the second stage is more closely defined by resistors R 3 and R 4 . Protection against polarity reversal is provided by diodes V 6 and V 7 and Zener diodes V 4 and V 5 operating in the forward mode. Protection against short term overload (e.g. ring-trip failure) is provided by V 4 , V 5 and R 5 causing either V 4 or V 5 to conduct before the current can reach a value sufficient to exceed the capabilities of V 3 . High voltage transients (e.g. due to lightning) is provided by resistor R 6 and Zener diodes V 4 and V 5 . Capacitor C 2 provides some protection against R.F.I.

Description

This invention relates to microphones, and in particular to a lone assembly having an electrical output similar to that of a carbon none and adapted to be fitted to a telephone subscriber's instrument.

According to one aspect of the invention there is provided a hone assembly adapted to be fitted to a telephone subscriber's instrument, iing a moving coil transducer, a two stage DC coupled amplifier coupled to the lucer output, and a reverse voltage and overvoltage protector circuit for acing the amplifier with the line, and in which the amplifier includes a transistor operated in the common emitter mode and directly coupled to a d and third transistor which second and third transistor form a compound n emitter output stage to the line.

According to another aspect of the invention there is provided a hone subscriber's instrument fitted with a microphone assembly as defined ie preceding paragraph.

An embodiment of the invention will now be described with reference le accompanying drawing in which Fig. 1 is a plan view of the microphone amplifier assembly, and Fig, 2 is a circuit diagram of the assembly of Fig. 1. - 2 42640 Referring to Fig. 1, the arrangement, which may be fitted to a telephone instrument, includes a moving coil transducer 11 together with an associated amplifier 12 both of which are mounted on a circuit board 13. The assembly is adapted to fit into a housing (not shown) which has dimensions corresponding to those of a standard telephone carbon transmitter. In particular the arrangement may be employed to replace the BPO No. 16 carbon granule transmitter in Post Office 700 type subset circuits.

The circuit shown in Fig. 2 includes an amplifier and a protection circuit.

I o Amplifier The amplifier consists of the transistors Vj, Vg and Vj. The transistors form a two stage dc coupled amplifier with full dc and partial ac feedback.

Transistor operates in the common emitter mode and causes signals from the microphone connected to its base to appear, amplified, across its collector resistor Rg. The signal then passes to the base of the compound common emitter stage formed by Vg and V3, the resulting amplified output appearing across Rg, Rg and the load connected across terminals Τη, Tg.

Overall dc feedback is provided by resistors Ρη and Rg, such that the dc voltage VCE3 appearing across V3 is defined by '1^ = V^ (βη + Rg) /Rg. where is the base-emitter voltage of νη.

In order to obtain sufficient gain from a two stage amplifier, the ac feedback provided by resistors Ρη and Rg is decoupled by capacitor Οη in the speech frequency band of interest (300Hz to 3,4kHz). Thus only local emitter ac feedback is operating for the first stage gain stabilization. The gain of the second stage is more closely defined by means of resistors R, and R^. The compound second stage is necessary in order that it shall be able to cope with the normal operating currents of 25 to 110mA and still provide a relatively high input impedance so as not to significantly load the first stage. -3 42640 Protection Circuit Protection against polarity reversal is provided by the diodes Vg and Vy and the zener diodes and Vg operating in their forward mode. Protection against short term overload such as 'Ring-trip failure' is provided by V^, Vg and Rg, the voltage drop across Rg causing either V4 or Vg to conduct before the current can reach a value sufficient to exceed the capabilities of Vg.

Protection against high voltage transients, such as that due to a strike by lightning is provided by resistor Rg and zener diodes V^ and Vg. Capacitor Cg provides some protection against the pickup of radio frequency interference.

Claims (3)

1. A microphone assembly adapted to be fitted to a telephone subscriber's instrument, including a moving coil transducer, a two stage DC coupled amplifier coupled to the transducer output, and a reverse voltage and overvoltage protector circuit for interfacing the amplifier with the line, and in which the amplifier includes a first transistor operated in the conmon emitter mode and directly coupled to a second and third transistor which second and third transistor form a compound common emitter output stage to the line.

2. A microphone assembly substantially as described herein with reference to the accompanying drawing.

3. , A telephone subscriber's instrument when fitted with a microphone assembly as claimed in Claim 1, 2 or 3.