Engineering Terms
"A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Transistor Amplifier Circuit Schematics

This page presents a number of different transistor circuits.
The first transistor circuit below is configured as a common Emitter.
A common Emitter circuit just means that the emitter lead is common to both the input and outputs circuits. In this case the emitter is grounded, so common to both input and output. The first example shows an NPN transistor followed by a PNP example, note that other than the transistor symbol changing, the circuits are identical.

BJT Inverter
Positive Output Swing

The basic transistor amplifier circuit used as an inverter showing just a collector resistor to set the current. Once the Base is made more positive than the emitter the transistor turns on and collector current flows. The output produced is positive with respect to ground. The 2N2222 NPN transistor is one of the more common general purpose transistors.

Common Emitter Transistor, NPN Inverter circuit
The input is applied to the Base of the transistor and the output taken from the transistor's Collector. The Emitter is ground or common, so an output is not possible from that terminal.

The same circuit as above now using a PNP transistor. Because the polarity is now reversed, a negative supply voltage is required at the collector producing a negative output with respect to ground. The 2N2904 PNP transistor is a common general purpose transistor.

BJT Inverter
Negative Output Swing

Common Emitter Transistor, PNP Inverter
Same circuit as above with a PNP transistor, note the negative voltage supply. So this output will switch between ground and some negative voltage.

This next example shows an Emitter Follower configuration. Basically a circuit that buffers the incoming signals and outputs a replica of the signal but slightly reduced in amplitude.

Emitter Follower Transistor Schematic
Emitter Follower Configuration
The output is a reduced version of the input due to the diode drop between the Base Emitter junction. The circuit has a high input impedance, so appears as a nice load for a low impedance source. The output has a low impedance, so makes a good source as a driver circuit. As a minimum circuit, only the transistor and Emitter resistor is shown.

The graphic shows an NPN transistor configured as an Emitter Follower, with a positive output swing.
A Negative output swing could be produced with a PNP transistor and negative Vcc.
Note that regardless of the transistor type, the output is taken from the same location. Some might label the circuit as a transistor amplifier because of the set-up, but the circuit will have a smaller output than the input and does not amplify the voltage of the input signal. However the circuit does amplify the input or Base current, by the Beta of the transistor.

The circuit to the right shows another example with both a Base resistor to set the base current and collector resistor which determines the current through the collector. The two capacitors C1 & C2 are normally used to block DC bias voltages from the circuits before and after the transistor.

BJT Amplifier

A dual output swing is possible but requires two supply voltages. Although the circuit shows the supply names as the actual voltages, its more common to call the collector voltage Vcc and the emitter voltage as Vee. Some Transistor Packages hold two transistors, both to keep the package count down and so they have similar temperature characteristics.

Transistor Amplifier, Dual Swing output [positive/negative]
The output of this transistor circuit sits between two transistors, one biased to a positive voltage and one to a negative voltage. So the output would than switch between to positive voltage and negative voltage.

PC motherboard

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