Class AB 20-W current booster is insensitive to temperature variations


Class AB amplifiers have been the workhorses of the audio world, thanks to their simplicity and ability to deliver high power levels with low distortion. But there are also a number of gremlins lurking in the AB architecture, primarily in the form of polarization instability and other heat related issues. I developed this circuit to solve some of the nasty flaws I encountered with Class AB power amplifier designs while retaining all of the virtues prized by engineers and audiophiles.

The standard topology of a Class AB power amplifier is shown in Figure 1a. When no signal is applied to the input, a relatively small current flows through the transistors to keep them in boost mode. This bias current is defined by the voltage drops across the terminals of diodes D1, D2, the base-emitter junctions of transistors Q1, Q2 and current detection resistors RCS1, RCS2.

Figure 1 Power amplifier topologies with diode bias (a) and operational amplifier bias (b).

The voltage drops at the four PN junctions do not match and they are all temperature dependent. Bias current is somewhat difficult to define and current adjustment is often necessary. To minimize these thermal effects, the amp’s diodes and transistors must be in close thermal contact to provide normal bias at varying output power.

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The circuit in Figure 1b solves bias issues. Since there are no temperature dependent components in the loop formed by R2, A1, RCS1, RCS2, A2 and R3, the bias current can be calculated accurately and is not temperature dependent. The diagram cited in Reference 1 used this topology to build a 2W audio amplifier with good performance.

This design idea (see Figure 2) pushes the improved topology to the power limit.

Figure 2 This current booster circuit delivers up to 20 W of power and has a well-defined quiescent current independent of temperature.

The key is OPA2991, a rail-to-rail I / O op amp capable of operating with a ± 20 V supply. Using small current sense resistors and two pairs of complementary transistors to deliver larger currents , the circuit can deliver up to 20W of power at an 8 Ω load with modest quiescent current, low distortion and high bandwidth. Unlike traditional Class AB designs, its bias current is temperature independent and no adjustment is required. Diodes D1 and D2 are on when the transistors are not driving, so the operational amplifiers are always in active mode.

Amplifier performance data highlights the benefits of its improved architecture:

  • The input resistance is 100 kΩ; it does not depend on the frequency
  • When VIN = 0, the output voltage is approximately 10 µV and the circuit draws 60 mA from the power supplies

figure 3 watch VIN (the yellow trace), VOUTSIDE (the green trace) and their spectra at maximum power and 1 kHz.

figure 3 VIN and VOUTSIDE at maximum power and 1 kHz. The upper graph shows the spectrum of VIN, the lower graph shows the spectrum of VOUTSIDE.

Other key points to remember:

  • The transfer coefficient is 0.99
  • The power delivered to the load is 20.4 W
  • The circuit draws 700 mA from the power supplies
  • Transistors Q2 and Q4 dissipate approximately 4W of power each, requiring appropriate heat sinks

The simulation shows that the circuit has high bandwidth and low distortion. At 1 kHz, the background noise is about 80 dB below the fundamental peak. The two spectra are quite identical, which means that the circuit does not introduce any distortion. The simulation reports a total harmonic distortion (THD) of 0.021%. Figure 4 illustrates these characteristics at 50 kHz.

Figure 4 VIN, VOUTSIDE and their spectra at 50 kHz.

At 50 kHz, the background noise is about 60 dB below the fundamental peak. The noise portions of the two spectra are quite identical. Near the fundamental peak (the first box in the graphs), the output spectrum is slightly higher than the input spectrum; this means that the circuit introduces some distortion into it. The simulation shows THD = 0.216%.


  1. Wenzel, C. High gain, fidelity audio amplifier. 05-02-2017.

–Jordan Dimitrov is an electrical engineer and doctor with 30 years of experience. He teaches electrical and electronics classes at a community college in Toronto.

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