Handbook for Sound Engineers

Consoles 937

either negligible or—better yet—an interesting effect on
the behavior of the circuit in response to stimulation.
Effectively biased—to avoid their turned-off regions—
FETs can have a square law response, transistors an
exponential (logarithmic) response of collector current
with respect to applied base voltage. Loga-
rithmic?—dBs are logarithmic!

25.16.2.1 Transistor Junctions

The departure point for the journey to VCA-dom is Fig.
25-107A, which is for our immediate purpose actually
pretty useless but elsewhere is known as a cascode
amplifier. The upper transistor’s emitter serves as a
nonvoltage varying load to the lower transistor, allowing
it to achieve large bandwidth current gain free of Miller
effect; the upper transistor (as essentially a
common-base amplifier) has no current gain but serves
to buffer the load in its collector from the lower tran-
sistor, which is busy doing all the work. Varying the
base voltage of the upper transistor has little effect on
anything other than altering the maximum voltage swing
capability on the load, certainly not gain, which is all
very much different from the long-tailed pair of Fig.
25-107B. Note that the upper stage is and can be used
differentially, as is the output, but it works single-ended
too. Here the current through the load is modified by
signals applied to either or both upper or lower stage
transistor bases. The overall current through the arrange-
ment is set by the lower transistor, which is shared by
the upper two; assuming both of the upper two tran-
sistor’s bases are held at the same voltage, the currents
will be shared equally; if one is raised with respect to the
other though, its share of the current will rise, having
stolen it from the other and vice versa (the total current
stays the same). So, wobbling the lower transistor’s base
will change the overall current, an upper’s base that in
both upper transistors, complimentarily, the combined
effect is multiplicative gain variation. (Conveniently,
one of the signals [usually the audio] can be applied to
and recovered from the pair of upper transistors differ-
entially, although it is not unusual for them to be driven
one-sided, the opposing base grounded.) The one
remaining drawback is that the operating points of all
the devices are moving around in accord with the
control voltage applied to the base of the lower transistor
and so the control voltage unavoidably appears as part
and parcel of the derived output signal.

25.16.2.2 Gilbert Cell

Fig. 25-107C shows what is the essential heart of a good

VCA—two long-tailed pairs back-to-back. Actually it’s a

bit more like three; a long-tailed pair with a long-tailed

pair in each output leg. So universal is this basic configu-

ration that it has become the Hoover of VCAs—it is

what springs to mind when VCA is mentioned; variations

and extensions to this theme are used extensively. Called

variously the Gilbert cell or, by RF guys, a

double-balanced modulator its main attributes are the

innate cancellation in the output of both the applied

Figure 25-107. VCA design.

V+

Output

Signal + Bias 2

Signal + Bias 1

V+

Multiplied

output

Signal 2 + bias

Signal 2

Signal 1 + bias

V+

Outputs

Current source

Control

Input

B. Long-tailed pair.

C. Gilbert cell or double-balanced multipler.

A. Cascade amplifier.