This General Electric circuit, in a documentation from the 1960s, allows the use of high resistance resistive sensors with unijunction transistors in a relaxation oscillator.
This amplifier with very high input impedance was suggested by Texas Instruments in a manual of...
Using only a ULN5804 integrated circuit, stepper motors up to 500 mA can be controlled from TTL...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....