LTspice, amplifier circuits.

This page contains several amplifiers for simulation with LTspice.
As always simulation will not show all aspects of the performance of the real circuit. There will be circuit parasitics which are not included in these simulations.
These include linear amplifiers (classes A, B and AB), nonlinear amplifiers (classes C and D), switched mode power amplifiers, and a variable gain amplifier.
Firstly you have to download and install LTspice from www.analog.com,
Then click on the names below to download the circuits and get LTspice to open the circuits
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).

Linear amplifiers tend to be inefficient.
Class A, class B, and class AB amplifiers are called linear amplifiers, "linear" means that the output volts is input volts x gain.
For a linerar amplifier, all of the various frequencies in the input signal will be present in the same ratio at the output.
An example of a class A amplifier is the common emmitter, however it is biased on even with zero signal, this means it is really inefficient (eg 10% efficient).
The class B amplifier is more efficient, but it introduces what is called crossover distortion, the class AB amplifier attempts to fix this crossover distortion while keeping the efficiency.
When the class B or class AB amplifier has zero signal, it is not biased on. But when it does have an input signal the difference between the voltage rail and the output signal is dissipated in the amplifier,
so efficiency is still probably less than 50%.

Class A, common emmitter design procedure.

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

Class D power Amps have their own section at the top level page of this website.
For these amplifiers the efficiency is higher than for the above linear amplifiers but the output is a distorted version of the input.
If for example the inut is a single frequency sine wave, then the output is more like a square wave at the same frequency, that is a load of harmonics of the input frequency have been intrioduced.
This can be okay if you are only worried about the one frequency, if the input is just one frequency, then even if the amplifier produces a load of harmonics you can filter them out. The gain of these amplifiers varies with amplitude, but the frequency is reliably reproduced, so if the input is a single varying frequency eg. FM, you can use these amplifiers.
If you want to use these types of amplifiers for AM, then you cannot just 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.
The below example uses class C, for Class D see the Class D power Amplifiers section at the top level page of this website.

A Switched mode audio frequency amplifier.

In terms of both linearity and efficiency this type of amplifier can be very good, so what's the catch, its that there has to be a switching frequency which is considerably greater than the highest signal frequency,
You need a filter to remove the switching frequency, and this filter needs a frequency band between the highest signal frequancy and the switching frequency.
This is okay if you are just amplifying audio, the highest audible freq is never above 20khz (and considerably lower than 20khz for people of my age), an SMPS can easily operate at 200khz, this leaves a decade between 20khz and 200khz for a low pass LC filter to operate.
So how do we modify a straightforward SMPS to become this type of amplifier.
A switched mode power supply (SMPS) is given a DC voltage as its reference. this type of amplifier is given the audio signal as its reference, depending on the exact circuit used, DC biasing of this audio signal may also be neccessary.

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

A variable gain amplifier is usefull for implementing AGC (automatic gain control).
While this method could be used to implement an AM transmitter, it is inefficient compared to modulating the audio onto the power rail.

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