Easy, Simple Voltage Controlled Amplifier (VCA) or Amplitude Modulator (AM)

It has taken ages, literally years of occasional pondering, for me to realize that there is a way to make a very simple voltage controlled amplifier that doesn't use complex non-linear circuits but rather uses simple resistors.
The advantages of doing it this way is that you can amplify both polarities of a carrier waveform with just one amplifier rather than using something like a push pull amplifier and complex transistor circuits. This also means you can add DC bias to your carrier wave as an alternative to using dual supplies because the circuit will not amplify it.
Fixed gain amplifiers or potentiometer controlled amplifiers have been my limit when creating analogue circuits but despite my knowledge about oscillators, filters and amplifiers in signal processing circuitry it seems silly that this has never occurred to me to try to do it this way.

In concept it is essentially voltage controlled resistance which is a brilliant concept when it comes to analogue electronics and there is probably more than one way to do it. For the method I will use it uses two components:

1. An LDR (light dependent resistor).
2. An LED or generic light bulb.

It requires no dual supply or any bipolar circuits such as a push pull amplifier.

The simple modulator circuit diagram.

You can see the two circuits involved which are galvanically decoupled, you can see the bulb's light shines on the LDR and changes its resistance proportionally to the light level. In my experience resistance will lower as light increases but it will depend on the component used. In this circuit the gain should decrease as light increases, it is a modification of a inverting amplifier.
The optical resistor method seems like it would have the best bandwidth and lowest slew rate assuming it has similar characteristics to fibre-optics but I will need to test this. I have used a bulb as it has a higher voltage range and can be driven from zero volts upwards where as an LED would need to be raised to around 1v before anything happens to the amplifier and would top off at around 2v. (The bulb may not even need the buffer circuit).
The two circuits will need to be kept in a dark environment so there is no interruption from natural light, but this is all it takes for a simple modulation circuit and I will undoubtedly go on to try and make voltage controlled oscillators next.

The output in this case would be a amplitude modulated carrier wave and would be good in an analogue synthesizer, simple AM transmitter or tremolo pedal etc.

See my 'Circuit Help Index' page for help with these circuits.