You can use a silicon diode with a fixed voltage drop of about 0.7V per diode.

The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance.

For output voltages above ten volts or so, ordinary silicon diodes are commonly used.

Unfortunately, the silicon diode's different characteristics caused the automatic gain control circuit to fail.

The silicon diodes have temperature dependency of -2mV per degree celsius.

The effect was soon demonstrated in ordinary silicon diodes and by the late 1960s oscillators at 340 GHz had been produced.

The X-rays are detected using a semiconductor detector, which is an array of silicon diodes.

However, the rapid development of the silicon diode made it ultimately redundant.

Also, the driving circuit can have silicon diodes mounted in thermal contact with the output transistors, to provide compensation.

However they were later replaced by silicon diodes with high efficiencies (close to 100% at high voltages).