Class A Audio Amplifier Impedance Matching


I designed a Class A audio amplifier and I’m not sure if it will work. Specifically, I’m not sure if I correctly calculated the right component values to do input and output impedance matching.

I assume 50ohm of impedance for the source and the transistor has a 1kOhm impedance with 8pF input capacitance. I calculate a capacitance of 58nF and inductance of 3mH for the L network.

I assume 50ohm of impedance for the load and the RFC has a inductance of 100uH with a resistance of 3.5ohms. I calculate a capacitance of 1uF and inductance of 170uH for the L network.



A variety of simulation tools such as LTspice and circuitlab are available at no charge, and can be very useful for studying such things. Take a few moments to enter the circuit shown into either, and you’ll find that the circuit shown probably isn’t going to work well as a general-purpose amplifier, regardless of the component values chosen.

For what it’s worth, audio seems to be a topic of fascination for many, and the quality of available information ranges from excellent to asinine. I’d suggest Douglas Self’s books as resources worthy of study.


Hello thebutterminecutter,

The schematic represents an amplifier appropriate for radio frequency, not audio.

For example:

  • 20 Hz the reactance of: C1 = 140,000 and L3 = 0.38 Ohms

  • 20 kHz the reactance of C1 = 140 and L3 = 380 Ohms

From this calculation we see that the “amplifier” acts as a filter with serious attenuation of low frequencies. It does not provide the desirable flat frequency response of an audio amplifier.

Inductors are not typically found in audio applications. Instead, we use resistors and select capacitors so that the reactance is low relative to the resistor values.

As a starting point, may I recommend:

Best Wishes,


P.S. The material written by Douglas Self is also very good.

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