Instead of using a simple collector resistor in a diff amp, gain can be increased through the use of an active load. Basically, this involves replacing the collector biasing resistor with a current mirror. The end result is that the effective AC collector resistance is now the much larger internal impedance of the current source, and this increases the voltage gain. Another way of looking at it is that the current mirror tries to maintain a fixed current in the collector. When we consider KCL at the at the output terminal, this means that any AC current variation must be forced out to the load instead of splitting between the load and the former collector biasing resistor.
References: Operational Amplifiers and Linear Integrated Circuits: Theory and Application; Chapter 1 section 6.
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12 сен 2024
