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The differential stage operates by way of comparing the signals between it’s inverting and non-inverting inputs and tries to make the difference equal to zero. This is where the differential input stage and -ve feedback network come in. Also note that the presence of a CCS minimises the variation in gain due to h ie, which becomes extremely small and can be neglected, and also makes the use of an emitter resistor unnecessary.Īlthough the non-linearity aspect of the CE stage is minimized, the infinite (or very large) gain needs to be brought down to a more useful level. 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). 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. We have a differential pair input stage, a voltage gain stage, an output (current gain) stage and a -ve feedback network. Let’s consider a typical VFB setup using BJT transistors. My headphone amplifier is a conventional VFB type employing commonly available parts. 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. It varies with the collector (or emitter) current and causes the gain to vary with the collector current, resulting in distortion. h ie is device dependent and highly non-linear. R c is the collector (or load) resistor which biases the transistor and R e is the emitter resistor which provides bias stabilization and local -ve feedback. h fe is the transistor current gain and h ie is its base-to-emitter resistance.
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With a simple common emitter amplifier, the gain is given by the formula (assuming h fe > 1) A v = h fe * R c /. 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. 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. A simple common emitter amplifier, for example, is not very linear and the overall gain is very much device dependent. I chose this class A topology, because it offers very good distortion figures without a lot of complexity. The design presented here is a 50mW power amplifier meant for phones with impedances of 32 Ohms and greater. This amplifier was born out of a need to use two sets headphones with my computer’s sound-card.