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The succeeding three stages to the delay line are shown below in Figure 2, a continuation of the amplifier in Figure 1.

(Click here for a larger view)
Figure 2 - Tek T935A 35 MHz Oscilloscope
Succeeding Stages to Input Buffer Amplifier

Q4258 and Q4268 form a fully differential amplifier, with a shared emitter resistance R4254, a 63.4 W, 1% resistor. Resistors R4257 and R4267 go to the –8 V supply and, being much larger than R4254, function as current sources to the BJT emitters.

The waveform at the base of the upper BJT is divided through the emitter circuit and is shared (nearly) equally with the lower BJT so that balanced waveforms having equal magnitude and opposite polarity appear at the load resistors. If R4254, or R_E, were split into two series resistors with a value of R_E/2 each, their midpoint would be a virtual ground for a balanced-input diff-amp. But for this stage, half the magnitude of the input waveform will appear instead, and is only applied to the upper BJT.

The next stage (Q4274, Q4284) is the second half—the common-base stage—of a complementary cascode amplifier. It is fully differential, as is the final common-emitter stage (Q4276, Q4286).

Stage Gain

To calculate the differential voltage gain of the complementary cascode stage, note that the emitter dynamic resistances of Q4274, Q4284, which shunt the resistors R4271, R4281 (both 825 W ), are much smaller, so that most of the dynamic current from Q4258, Q4268 flows through Q4274, Q4284 to develop a voltage across R4273, R4283 (both 499 W ).

where the upper and lower voltages are denoted by subscripts u and l. Their differences are the input and output differential voltages. Each BJT contributes to the total gain—hence the ×2 before the BJT gain in A_v. Because R_E (R4254) is so close to the value of the dynamic emitter resistance of the BJT, r_e, a better gain approximation adds 2·r_e to R_E in the denominator of the gain equation, where

at room temperature. Then A_v � –12.9, with 3.72 mA of emitter current for each BJT. The loading of the input impedance of the next stage on the load resistors is neglected. Do you suppose the amplifier designer was shooting for a gain of –10?

Conclusion

Fully differential monolithic amplifiers such as the ADI AD8138 are now appearing to drive high-resolution A/D converters and for other high-performance (high-speed and high-precision) amplifier applications. Their predecessors can be found in typical oscilloscope circuitry from the last few decades.

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