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A 50mW Class-A Headphone Amplifier

A 50mW Class-A Headphone Amplifier

This amplifier was born out of a need to use two sets headphones with my computer sound-card. The design presented here is a 50mW power amplifier meant for phones with impedances of 32 Ohms and greater.
I chose this class A topology, because it offers very good distortion figures without a lot of complexity. A simple common emitter amplifier, for example, is not very linear and the overall gain is very much device dependent. In the case of my amp, it uses a voltage feedback (VFB) topology, and the gain is dependent only on the ratio of 2 resistors. Plus, the amp has very good power supply and common mode rejection on account of the differential input pair and the current source used to bias it.

With a simple common emitter amplifier, the gain is given by the formula (assuming hfe >> 1) Av = hfe * Rc / [(hfe * Re + hie)]. hfe is the transistor current gain and hie is its base-to-emitter resistance. Rc is the collector (or load) resistor which biases the transistor and Re is the emitter resistor which provides bias stabilisation and local -ve feedback. hie is device dependent and highly non-linear. It varies with the collector (or emitter) current and causes the gain to vary with the collector current, resulting in distortion. If Re is large enough to make hie negligible, then Rc will also need to be large and the amplifier won’t be able to source/sink current into low-impedance loads.

My headphone amplifier is a conventional VFB type employing commonly available parts. Let’s consider a typical VFB setup using BJT transistors. We have a differential pair input stage, a voltage gain stage, an output (current gain) stage and a -ve feedback network. For the explanation, I will not include the current gain stage since it has no role to play as far as the voltage gain is concerned. The voltage gain stage is a CE stage with a constant current source (CCS) for the collector resistor and theoretically has an infinite voltage gain as per the above formula (the output impedance of a true CCS is infinite). Also note that the presence of a CCS minimises the variation in gain due to hie, which becomes extremely small and can be neglected, and also makes the use of an emitter resistor unnecessary.

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