Page 107 - The Art and Science of Analog Circuit Design

One Trip Down the 1C Development Road

Figure 8-4.
Class AB current
feedback fader.

SOOjaA

current levels were set based on previous current feedback amplifiers
already designed. It was time to proceed to the control section.
For linear control of the currents being steered by a differential pair,
the voltage at the bases of the steering transistors must have a nonlinear
characteristic. This TANK characteristic is easily generated with "pre-
distortion" diodes. The only requirement is that the currents feeding the
diodes must be in the same ratio as the currents to be steered. The ckcuit
of Figure 8-5 takes two input control currents, K and (1-K), and uses Ql
and Q2 as the pre-distortion diodes to generate the control signal V CN for
the NPN steering transistors. The collector currents of Ql and Q2 then
feed the pre-distortion diodes Q3 and Q4 that generate V cp to control the
PNP steering transistors.
I noticed that the linearity of the signal gain versus diode current is
strongly influenced by the bulk R b and R e of the current steering tran-
sistors. After consulting some papers on multipliers (thank you Barry
Gilbert) I found that there are some topologies where the bulk R^ and R e
of the pre-distortion diodes compensate the equivalent in the steering

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