Bipolar Transistor Amplifiers

Amplifier Configurations

One of the main applications of a bipolar junction transistor (BJT) is in amplifier circuits. The circuit configuration specifies which electrodes in the amplifier are used for input and output signals. There are three configurations of a BJT amplifier circuit: common-emitter (CE), common-collector (CC) and common-base (CB). The configuration is named for the electrode that is common for input and output networks. The CE is the most widely used for amplifiers because it has the best combination of current gain and voltage gain.

Transistor Biasing

For the transistor to operate properly as an amplifier, the base-emitter junction should be forward-biased and the base-collector junction – reverse-biased. This is called forward-reverse bias. The three dc voltages for the biased transistor are the emitter voltage UE, the collector voltage UC and the base voltage UB. These voltages are measured with respect to ground. The formulas for their calculation are given in the figure.

Voltage-Divider Biasing (VDB)

The voltage-divider bias (VDB) configuration uses only a single dc source to provide forward-reverse bias to the transistor. Resistors R1 and R2 form a voltage divider that provides the base bias voltage UB. Resistor RE allows the emitter to rise above the ground potential. The dc voltages with respect to ground - UE, UC and UB are calculated as shown in the figure.

Play & Learn Voltage-Divider Biasing (VDB)

Calculate UB, UE and UC. What is the collector-emitter voltage UCE if UCC = 10 V? Use the voltmeter to verify your decision. Try to make relation between schematic components and corresponding components on the printed circuit board (PCB) implementation of the same schematic.

Load Line

When the transistor is cutoff, there is essentially no collector current and UCE = UCC. When the transistor is saturated UCE is approximately zero. Thus all the UCC is dropped across resistors RC + RE. In this conditions IC reaches the maximum possible current for the circuit – saturation current IC(sat).

A straight line drawn on the collector curves between the cutoff and saturation point of a transistor is called the load line. Notice that the load line is determined only by the resistors RC, RE and UCC and not by the transistor itself.

Operating Point

The base current IB is established by the base bias. The intersection point between the collector current curve (at this IB) and the dc load line is called the quiescent or Q-point or operating point. The coordinates of the Q-point are the values for IC and UCE at that point, as illustrated.

When the resistance R1 varies, it causes IB to vary. This makes IC and UCE change and the operation point moves along the load line. The load line contains every possible operation point for the circuit.

Play & Learn Load Line and Operating Point

Change the supply voltage through the potentiometer RS. Modify the values of resistors R1, and RC. Observe the differences in the load line slope and load line position as well as the variation of the dc operation point along the load line. Explain the influence of each variable parameter.

Troubleshooting

Determine if there is a fault in the circuit. If so, identify the faulty component.

Coupling an AC Signal into a DC Bias Network

After an amplifier has been biased to the proper operating point Q, a small ac voltage can be applied to the input. A coupling capacitor allows an ac signal to be coupled into an amplifier without disturbing its Q point. The capacitor acts as an open to dc and as a short to ac. In order for the ac signal voltage to be passed through the capacitor without being reduced, capacitor's reactance should be much smaller (at least 10 times) than the resistance at the lowest frequency of operation.

Common-Emitter Amplifier

The common-emitter amplifier is a configuration in which the emitter is at ac ground as shown in the figure. The input signal is applied to the base, and the output signal is taken from the collector. C1 and C2 are coupling capacitors used to pass the ac signal into and out of the amplifier. As a result, the source or load will not affect the dc bias voltages. CE is a bypass capacitor that shorts the emitter ac signal voltage to ground without disturbing the dc emitter voltage. All capacitors should have a reactance of approximately zero at the signal frequency.

Play & Learn CE Voltage Gain

Measure the amplitude of the input and output ac signals by using the oscilloscope settings. Calculate the amplifier voltage gain. Disconnect the bridging plug (jumper) to the bypass capacitor CE. Measure the output voltage again. How does the voltage gain change?

Explain why the voltage gain changes when the bypass capacitor is disconnected.

Waveforms & DC Levels

An input signal with an amplitude of 50 mV is applied to the CE amplifier. What will be the output signal voltage, if the amplifier voltage gain Au = 10? Determine the dc collector voltage at which the output signal voltage is riding. Check your decision by observing the input and output waveforms. Remember that lowercase italic letters and subscripts indicate ac signals.

AC Load Resistance

When the load RL is connected in the output through a coupling capacitor, the total ac load resistance is the parallel combination of RC and RL. This total resistance is also called the ac collector resistance, symbolized by rc.

The voltage gain of the CE amplifier equals the ac collector resistance divided by the internal emitter resistance of the transistor re (not shown in the schematic). Notice that the voltage gain for an unloaded amplifier Au = RC / re has a greater value since RC > rc.

Signal Operation on the Load Line

The amplifier's operation can be represented by signal variation on output characteristics with a load line, as shown in the figure. The input signal varies the base current above and below its dc value. This causes much larger variation in the collector current because of the transistor current gain. The variation in collector current produces a corresponding variation in the voltage across RC. As a result, the collector-emitter voltage UCE also changes. The operation is linear when the shape of the output signal is an amplified copy of the input signal.

Two Load Lines

Every amplifier has a dc equivalent circuit and an ac equivalent circuit. Because of this, it has two load lines: a dc load line and an ac load line. With the emitter at ac ground RE has no effect on the ac operation. Furthermore, the ac collector resistance rc is less than the dc collector resistance RC. For this reason the ac load line has a higher slope than the dc load line. The peak-to-peak sinusoidal current and voltage are determined by the ac load line.

Multistage Amplifiers

Several amplifiers can be connected in a cascaded arrangement with the output of one amplifier driving the input to the next. The purpose is to increase the overall gain. Each amplifier in the cascaded arrangement is known as a stage. The overall multistage gain is the product of the individual loaded voltage gains. The figure shows an example of a two-stage amplifier of identical CE stages. Since the input resistance of the second stage presents an ac load to the first stage, the voltage gain of the first stage is reduced.

Class A

If the output signal takes up only a small percentage of the total load line, the amplifier is a small-signal amplifier. When the output signal approaches the limits of the load line, the amplifier is a large-signal amplifier.

An amplifier is a class A amplifier if it is biased such that it always operates in the linear region where the shape of the output signal is an amplified copy of the input signal. For small-signal operation, the location of the Q point is not critical. But with large-signal amplifiers, the Q point has to be at the middle of an ac load line to get the maximum possible output swing.

Clipping Large Signals

If the Q point is not centered, the output signal is limited. If the Q-point is moved higher or lower, a large signal will drive the transistor into saturation or cutoff. Thus saturation or cutoff clipping occurs. Both types of clipping are undesirable because they distort the signal. When a distorted signal drives a loud speaker, it sounds terrible.

Max Amplitude

A non-centered Q-point limits output swing. If the Q-point is above the center, the maximum peak (MP) output is UCEQ. When the Q-point is below the center, the maximum peak output is ICQ.rc. For any Q-point, the maximum peak output is the smaller of these two values.

Determine the maximum undistorted output voltage for the circuit. Measure dc voltages to find Q-point values ICQ and UCEQ.

Class B

The class B amplifier is biased at cutoff and clips half the cycle. The primary advantage of class B over class A is improved efficiency - it permits more output power for a given amount of input power.

The figure shows one type of push-pull class B amplifier using two emitter followers. Push-pull means that one transistor (NPN) conducts for half a cycle while the other (PNP) is off, and vice versa. There is no dc base bias (UB = 0). The input signal voltage must exceed UBE before a transistor conducts. As a result, the output is distorted as shown in figure.

Biasing

To eliminate distortions, both transistors in the push-pull arrangement must be biased slightly above cutoff when there is no signal. This can be done with the voltage divider and diode arrangement as shown in the figure. As a result, the collector-emitter voltages in the Q-point for both transistors are UCC/2. During the signal alternation of the input signal, the output voltage is driven from the Q-point value to near UCC or to near zero, producing peak voltage approximately equal to UCC /2.

Class C

The class C amplifier conducts less than half a cycle. A basic CE class C amplifier is biased below cutoff value with the – UBB supply. The ac source voltage has a peak value that is slightly greater than UBB + UBE. The transistor is on for a short time and off for the rest of the input cycle. Because the output voltage is cruelly distorted, class C amplifiers use a resonant circuit as a load. A parallel resonant circuit can filter the pulses of IC and produce a sine wave of output voltage. This is why class C amplifiers are used as tuned amplifiers at radio frequencies (RF).

Amplifier Troubleshooting – 1

Determine if the circuit is operating properly. If not, identify the fault. For correct amplifier circuit dc bias values are as follows: UB = 1.8 V, UC = 6.04V, UE = 1.1 V. Voltage gain is 120.

Amplifier Troubleshooting – 2

Determine if the circuit is operating properly. If not, identify the fault. For correct amplifier circuit dc bias values are as follows: UB = 1.8 V, UC = 6.04V, UE = 1.1 V. Voltage gain is 120.

Amplifier Troubleshooting – 3

Determine if the circuit is operating properly. If not, identify the fault. For correct amplifier circuit dc bias values are as follows: UB = 1.8 V, UC = 6.04V, UE = 1.1 V. Voltage gain is 120.

Amplifier Troubleshooting – 4

Determine if the circuit is operating properly. If not, identify the fault. For correct amplifier circuit dc bias values are as follows: UB = 1.8 V, UC = 6.04V, UE = 1.1 V. Voltage gain is 120.