Purpose of PNP transistor in ultrasonic driver circuit
What is the purpose of Q2, R2, C1 and C6 in the circuit?


Together the form a cascode common base amplifier. Their job is to add gain but also isolate the emitter of Q2 from capacitive loading. In a real sense Q2 extends the bandwidth of this circuit. In this circuit Q1 is just a input buffer, so Q2 has the initial gain.

R2 is bias and limits the base current, while C2 acts as an AC couple to ground. C6 with L1 act as a low pass filter, likely to prevent side-band harmonics from passing to the output stage.

Its origin goes back to the days of tubes when bandwidth was tough to get. Also high-performance audio amplifiers use them for the same reason at the input stage.

In this case Q2 is referenced to ground, but often when higher voltages are used it is biased at 1/2 Vcc. This cuts down on the collector voltage that Q2 sees.

This cascode topology is helpful in RF circuits where resistors are used instead of constant-current sources or current mirrors, as they have more capacitive loading then a resistor.

In high performance audio they toss in everything that helps. Cascode with cascade design plus current mirrors to get maximum bandwidth and voltage swing. Some of these amplifiers have supply rails of +/- 100 volts, so a cascode input and driver stage help divide up such a high voltage.

The key for cascode design is the lower voltage transistor driving the common base transistor never sees a voltage higher than the common base transistor has at its emitter, which is fixed because its base voltage has a fixed reference.