Bipolar Junction Transistors

The Basic Transistor Amplifier

Before going into the basic transistor amplifier, there are two terms you should be familiar with: amplification and amplifier. Amplification is the process of increasing the strength of a signal. A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. An amplifier is the device that provides amplification (the increase in current, voltage, or power of a signal) without appreciably altering the original signal.

Transistors are frequently used as amplifiers. Some transistor circuits are current amplifiers, with a small load resistance; other circuits are designed for voltage amplification and have a high load resistance; others amplify power.

Now take a look at the NPN version of the basic transistor amplifier in the figure above and let's see just how it works.

By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitter-base battery eliminated. In addition to eliminating the battery, some of these biasing methods compensate for slight variations in transistor characteristics and changes in transistor conduction resulting from temperature irregularities. Notice in the figure above that the emitter-base battery has been eliminated and the bias resistor R_B has been inserted between the collector and the base. Resistor R_B provides the necessary forward bias for the emitter-base junction. Electron current flows in the emitter-base bias circuit from ground to the emitter, out the base lead, and through R_B to V_CC. Since the current in the base circuit is very small (a few tens microamperes) and the forward resistance of the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of the transistor. However, this is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor.

With Q₁ properly biased, direct current flows continuously, with or without an input signal, throughout the entire circuit. The direct current flowing through the circuit develops more than just base bias; it also develops the collector voltage (V_C) as it flows through Q₁ and R_L. Notice the collector voltage on the output graph. Since it is present in the circuit without an input signal, then the output signal starts at the V_C level and either increases or decreases. These DC voltages and currents that exist in the circuit before the application of a signal are known as quiescent voltages and currents (the quiescent state of the circuit).

Resistor R_L, the collector load resistor, is placed in the circuit to keep the full effect of the collector supply voltage off the collector. This permits the collector voltage (V_C) to change with an input signal, which in turn allows the transistor to amplify voltage. Without R_L in the circuit, the voltage on the collector would always be equal to V_CC.

The coupling capacitor (C₁) is another new addition to the transistor circuit. It is used to pass the AC input signal and block the DC voltage from the preceding circuit. This prevents DC in the circuitry on the left of the coupling capacitor from affecting the bias on Q₁. The coupling capacitor also blocks the bias of Q₁ from reaching the input signal source.

The input to the amplifier is a sine wave that varies tens of millivolts above and below zero. It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter. As the input signal goes positive, the voltage across the emitter-base junction becomes more positive. This in effect increases forward bias, which causes base current to increase at the same rate as that of the input sine wave. Emitter and collector currents also increase but much more than the base current. With an increase in collector current, more voltage is developed across R_L. Since the voltage across R_L and the voltage across Q₁ (collector to emitter) must add up to V_CC, an increase in voltage across R_L results in an equal decrease in voltage across Q₁. Therefore, the output voltage from the amplifier, taken at the collector of Q₁ with respect to the emitter, is a negative alternation of voltage that is larger than the input, but has the same sine wave characteristics.

During the negative alternation of the input, the input signal opposes the forward bias. This action decreases base current, which results in a decrease in both emitter and collector currents. The decrease in current through R_L decreases its voltage drop and causes the voltage across the transistor to rise along with the output voltage. Therefore, the output for the negative alternation of the input is a positive alternation of voltage that is larger than the input but has the same sine wave characteristics.

By examining both input and output signals for one complete alternation of the input, we can see that the output of the amplifier is an exact reproduction of the input except for the reversal in polarity and the increased amplitude (tens of millivolts as compared to a few volts).

The PNP version of this amplifier is shown above. The primary difference between the NPN and PNP amplifier is the polarity of the source voltage. With a negative V_CC, the PNP base voltage is slightly negative with respect to ground, which provides the necessary forward bias condition between the emitter and base.

When the PNP input signal goes positive, it opposes the forward bias of the transistor. This action cancels some of the negative voltage across the emitter-base junction, which reduces the current through the transistor. Therefore, the voltage across the load resistor decreases, and the voltage across the transistor increases. Since V_CC is negative, the voltage on the collector (V_C) goes in a negative direction (as shown on the output graph) toward V_CC (for example, from -5 volts to -7 volts). Thus, the output is a negative alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has a larger amplitude.

During the negative alternation of the input signal, the transistor current increases because the input voltage aids the forward bias. Therefore, the voltage across R_L increases, and consequently, the voltage across the transistor decreases or goes in a positive direction (for example: from -5 volts to -3 volts). This action results in a positive output voltage, which has the same characteristics as the input except that it has been amplified and the polarity is reversed.

In summary, the input signals in the preceding circuits were amplified because the small change in base current caused a large change in collector current. And, by placing resistor R_L in series with the collector, voltage amplification was achieved.