12.4.3 Circuit Description
Each amplifier acts as a voltage to current converter. In the following
description the circuit reference of the “a” – “A” amplifier are used. The base of
transistor VT1 is biased to about –12.5V by a potential divider R3, R4 across the –
20V supply. An emitter load R6, 6.8k
Ω
and a collector load R5, 27k
Ω
are provided
and the output is taken through a coupling capacitor C3. It will therefore be seen
that the amplifiers will feed a low impedance load connected to the output with 1/6.8
mA per input signal volt from an impedance of 27k
Ω
. In order that the gain of an
Amplifier B shall be within 0.1 dB of any other Amplifier B, the tolerance on the
value of emitter resistor R6 is specified at
±
1/2%.
12.4.3 Output
Amplifiers B are used throughout the Mixer for mixing purposes and they are
always followed by an Amplifier C which has a virtual earth input. Any number of
Amplifiers B can be connected simultaneously to an Amplifier C. The amplifier will
deliver at least 0.4mA r.m.s without clipping.
12.5 Amplifier Type C
12.5.1 Versions
There are two versions of the Amplifier C Type C2, when fed from an
Amplifier B, gives unity overall voltage gain. Type C1, when used in conjunction with
a control to Drawing No. A40 has unity gain when the control is in the central
position. This control can increase and decrease the gain by 10dB in 2dB steps.
12.5.2 Circuit
The circuit diagram is shown on Drawing No. AE.205.
12.5.3 Circuit Description
The circuit comprises two transistors, VT1 is connected as a high gain
amplifying stage and VT2 is an emitter follower output stage. The two transistors
are d.c. coupled and d.c. coupled feedback is provided by R5 between the output
emitter and the input base. Input and output coupling capacitors are provided. C2
and R6 in the forward path and C3 and R4 in the feedback path serve to prevent r.f.
oscillation. The table shows the values of R2 and R5 which are different in the two
versions. In the C2 version the value of the feedback resistor R5 is 6.8k
Ω
, this
being the value of the emitter resistor in the Amplifier B. This results in the
Amplifiers C2 giving 6.8 volts out per milliampere in so that a B + C2 combination
gives overall a voltage gain of unity. When the Amplifier C1 is used the A40 control
is connected between the output and the input. It is therefore effectively in parallel
with the feedback resistor and its resistance values are such that, in parallel with R5
(47k
Ω
) the required 2dB steps of gain are obtained.
