12.22 Limiter
12.22.1
Purpose
The so-called limiter is provided with a switch which enables it to be used
either as a 2:1 compressor or as a limiter with up to 20dB of compression or limiting.
12.22.2
Circuit
The circuit diagram is shown in Drawing No. CE226.
12.22.3
Circuit Description
The input is coupled by C1 to a pair of transistors VT1, VT2 connected as a
direct coupled pair with direct coupled feedback to give a gain of 14dB. The base of
VT1 is held at about –16V by R2 and R3 which, together with R1, provide an input
impedance of 20k
Ω
. The positive and negative supplies to this pair are decoupled
by R7, C2 and R8, C3 respectively.
The output of this pair is coupled to the base of VT3, an emitter follower, the
emitter of which is connected to the base of VT6. VT5 and VT6 form a long tail pair,
the tail being mainly the collector impedance of VT24.
Zener diodes have the property that their impedance is approximately a
logarithmic function of the current. Two diodes are provided in each of the collector
circuits of VT5 and VT6. The voltage gain of each of these transistors is therefore
the ratio of the impedance of the two zener diodes in the collector circuit to the
impedance in the emitter circuit, R10 or R20. The impedance of these zener diodes
is determined by a control current supplied by VT24.
The total signal at the collectors of VT5 and VT6 therefore contains the
desired signal in push-pull together with an undesired control signal in push-push. It
is most important that no appreciable amount of control signal shall appear in push-
pull. To realise this the diodes are selected as matched pairs before being fitted.
D1 and D3 are one matched pair, and D2 and D4 are another matched pair and
these components must only be replaced as matched pairs.
Although only VT6 is directly fed with signal, in order to maintain the balance
of VT5 and VT6 with changes of temperature, it is necessary to provide these two
transistors with similar bias supplies. The base of VT5 is therefore fed from an
emitter follower, having base and emitter resistances equal to those of VT3. The
bias chain is common to the two transistors as far as possible. R 12 and R16 being
common. For initial balancing AOT7 and AOT9 are provided. In order to avoid the
need for high values of the AOT resistors, R13 and R14 are made much less than
R10 and R11. In order to equalise the resistances seen by the bases of VT3 and
VT4, R15 is added. The transmission of signal to the base of VT4 is prevented by
C5. In order further to improve the matching of the pairs of zener diodes, at the low
impedance (high control current) end of their range of operation, R19 and AOT8 and
10 are included. If any component which affects the balance of VT5 and VT6 has to
be replaced, the setting-up procedure for AOT7, 8, 9 and 10 must be carried out.
