PWM amplifier using a two-quadrant drive circuit

Article By : Darshil Patel

This PWM amplifier circuit has a pretty good response at higher frequencies.

Presented here is a PWM amplifier circuit (Figure 1) built around a two quadrant driver circuit that can amplify 3.3V or 5V signals to a higher level. 

Figure 1 PWM amplifier circuit built around a two quadrant driver circuit.

Here, R1 and C1 make up a modest filter for transistor Q4. 3.3V at the emitter of Q5 will reverse bias base-emitter junctions and will make the current flow through R11 to the base of Q4. Current flowing through Q4 drives Q3 and Q2 and the current flow depends on the PWM level. As the PWM approaches 3.3V, less current flows from Q4 which makes Q3 inactive and Q2 will sink the current. At 0V, 210uA of current flows through Q4 which will turn-ON the pull-up transistor Q3. Thus, the output signal from the two quadrant driver is inverted.

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Three degeneration resistors for the three bipolar transistors (BJTs) are added, which will minimize the maximum current at the output and also they will nip out the shoot-through at PWM signal edges. Further, they also provide some output short circuit protection.

C2 and C3 are added, which will further reduce the shoot-through. The PWM output from two-quadrant driver being inverted, a common source stage with degeneration is employed which will provide us amplified reflection of our original PWM signals. One can add a buffer at the output to drive loads like RC analog PWM converter, etc.

Response of the circuit at 500 Hz:

At low frequencies, the response is Excellent, the edges are fast and transition is as fast as the original PWM signals. Further, delay is negligible.

Response at 10 kHz:

At 10 kHz, one can observe the propagation delay and also the transition is not that fast now.

Response at 50 kHz:

At 50 kHz, the response is getting worse but still not so bad. Here, there is a dip at the turn-OFF transient and also propagation delay is quite significant.

Thus, the results shows us that the circuit has pretty good behavior at the frequencies up to 10-20 kHz but at higher frequencies, the response though bit degraded, still there is no shoot-through and other noisy behaviors.

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