Voltage to current converter (V-I converter)

This circuit is one of the simplest voltage to current converters to drive low loads. In this example a small 0,25 Watts speaker, with 8Ω resistance, is driven. It is suitable for microcontroller designs or any other circuit that uses PWM (TTL logic). You can use it for analog signals as well.

Details

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The circuit is not amplifying the input voltage signal (use a preamp if it is necessary). For analog signals is better to use a power supply higher than the maximum voltage of the input signal, to avoid saturation effects. For digital signals it is not necessary.
The equation applied to this circuit is I = ^Vin/_R (where R is 22 Ohms, and I the current at the transistor's emitter).

Schematic Diagram

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Here is the schematic diagram of the voltage to current converter.

Attention: Vcc is the power supply (5 or 9 Volts). You can also use a 4.5 Volt battery. Do not use a 22 Ohm 1/4 Watt resistor. Use a 0.5 Watt resistor instead.