Distortion in output signal in amplifier after connecting load

Question

I want to design an amplifier which works in 20kHz-100kHz range with 0.1V peak to peak signal.

This is my circuit.

It works fine in the simulation. It works fine without connecting R6 8 ohm load. But when I connect the 8 ohm load output signal get distorted.

How can I solve this problem?

Answer 1

Try lowering the values of the 10k resistors ( R3 & R7). The 10k values may not be able to supply enough base current to the driver transistors.

Answer 2

Imbalance of base-drive will cause different voltages across the DC_blocking capacitors. Examine the voltages at bases of your 2 Darlingtons.

Answer 3

This can be easily solved with NFB (neg.feedback) using an Op Amp with some low gain but adequate GBW or a better CE amplifier with RC feedback from the output to the input with a series R added from source to achieve about half of the expected gain from R ratios. also adding Re=5 improves THD yet reduces gain to 112/5. Define desired gain better to choose.

Consider Rs=100 added or included and 2k for NFB from output to cap to base for a maybe gain of 10.

Each TIP on the datasheet has an hFE = 25 typ. @ 3A and approx =130 @ 3A/25A or a net gain of In order to have high Damping factor >>100 the Zout must be <<1% of the 8 ohm load.

• since 2 stages only yields a total hFEt=3250 x 8 ohm load , Req= 8*3250 = 26k for maximum power transfer, but for damping the source impedance must be <<1% of this or 260 ohms which is near the open loop Rout of many Op Amps.
  • Your CE amp has an output R=122 ought to be OK
• But with NFB using the inverting feedback of an Op Amp this can be reduced by Gain.
• Stray capacitance can cause loss of phase margin so an RC snubber is used to attenuation potential oscillations at low gain.
• so choice of Op Amp with high GBW is preferred for fmax in your passband is desirable, e.g. >=10MHz and Av=<10?

Then your crossover distortion reduces by amount feedback gain e.g. 10MHz/100kHz/Av=10=10

You must define your design goals for THD and gain with f first.