What can be used as an amplifier?

Answer 1

Lots of things can be amplifiers. They can be as simple as a single transistor.

Picking an amplifier requires a bit of system knowledge. What's the signal to be amplified? What is the output delivered to? Should the "DC" bias levels be decoupled?

Here's an example: An audio amplifier is typically a voltage-in, power-out device. So, the input should have high impedance, while the output has low impedance. The device should be powerful enough to handle large currents and power levels.

A radio-frequency amplifier is typically power-in, power-out (with impedances matched to 50 Ohms to avoid signal reflection).

A typical blood glucose sensor typically has a current output. So, connecting it to a microcontroller and displaying the result would require a current-to-voltage amplifier.

For most of these applications, it is possible to adapt an operational amplifier to create the desired amplification (trans-impedance, trans-conductance, etc.). Sometimes, when higher performance is required, it's necessary to build a circuit using OTA's (operational transconductance amplifiers) and transistors (common-source, common-emitter, common-base, mirrors, folded cascodes, etc). It's usually easier to find a special-purpose one to buy (e.g., buy an audio amp, or a RF amp, or an instrumentation amp, etc from Digikey).

Hope this helps!