PUT voltage controlled sawtooth generator

A simple voltage controlled current source can be made using a current mirror. For more info, click on current mirrors or current sources.
In the circuit below, the current mirror is formed by Q3 and Q4.
The control voltage is converted to a reference current, that is flowing from the power supply via the base-emitter junction of Q3 and through R4.
Q4 mirrors a copy of this current, that is then used to charge C1. The lower the control voltage, the higher the current that is used to charge C1. So the higher the frequency of the sawtooth waveform.
The frequency varies linearly with the control voltage. The control voltage can not be as high as the power supply because the transistors of the current mirror need at least 0.7V to conduct.