Amplifiers

This section contains several types of circuit blocks for simulation with LTspice.
These include linear amplifiers (classes A, B and AB), nonlinear amplifiers (calsses C and D), switdhed mode power amplifiers, and a variable gain amplifier.
Firstly you have to download and install LTspice from www.analog.com, you need this free software to open the circuits,
Then click on the names below to download the circuits and get LTspice to open the circuits
Regarding text on the circuit diagrams, blue text is just comment, black text is read by the spice simulator.
There is comment text on the circuits explaining how to run the simulation and plot the waveforms.
In case of difficulty downloading, the files in this section are in a ZIP file here.

The circuits in this section as a ZIP file.

Linear Amplifiers (class A, Class B, and class AB, amplifiers).

Class A, class B, and class AB amplifiers are called linear amplifiers, to a first approximation the output volts is input volts x gain.
An example of a class A amplifier is the common emmitter, it is biased on even with zero signal, and is hence really inefficient (eg 10% efficient).
The class B amplifier increases efficiency but it introduces what is called crossover distortion, the class AB amplifier attempts to fix this while maintaining the efficiency. The class B or class AB amplifier is not biased on with zero signal, but with a signal the difference between the voltage rail and the output signal is dissipated in the amplifier, so efficiency is still probably less than 50%.

Nonlinear Amplifiers (class C, and class D, amplifiers)

For these amplifiers the efficiency is higher than for the above linear amplifiers but the output is a distorted version of the input.
The gain of these amplifiers varies with amplitude, but if the input is a single varying frequency then the frequency is reliably reproduced, this means that you can use these amplifiers for FM.
If you want to use these types of amplifiers for AM, then you cannot put the AM signal into the input, as the amplifier would distort it, you have to put the carrier into the input and modulate the audio onto the supply rail.

A Switched mode audio frequency amplifier.

This type of amplifier is only usefull for lower frequencies (eg audio), it has similarities to a switched mode power supply.
A switched mode power supply (SMPS) is given a Dc voltage as its reference. we swap this DC reference for a low frequency AC reference, or an AC + DC reference.
If the switching frequency is at eg 100khz, then the output filter which we need to remove the 100khz from the output, will have to start cutting off above say 20khz, then 20khz is the maximum signal frequency.

Varying the gain of an Amplifier, for AGC (Automatic gain control).

A variable gain amplifier is usefull for implementing AGC (automatic gain control).
It could also be used to implement an AM transmitter, but modulating the audio onto the power rail can be more efficient.

After a few circuits you may be able to ignore the directions written on the cct diagrams, as LTspice is reasonably simple to operate.