Bipolar Junction Transistor Circuits:Circuit Applications of the BJT

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Circuit Applications of the BJT

Two very common applications of the BJT are the linear amplifier and the digital or switching circuit. Many signals of interest are too small to be useful in some desired function. Signals such as the output of a microphone or the output of a radio antenna are in the microvolt to millivolt range. To drive a speaker or to drive a radio receiver, these signals must be amplified to levels of several volts. Thus, amplification is an important function in electronics.

Digital circuits are used to represent binary numbers and to perform logic operations and numerical calculations with binary numbers. The simplest logic circuit must accept two different voltage levels that signify a binary “1” or a binary “0” and produce an appropriate output voltage at either of the two acceptable levels. Digital circuits are often based on the simple BJT switch to be discussed in the following paragraphs.

Voltage Amplification

Since collector current is always a factor of b times the base current when the BJT operates in its active region, current amplification can be quite high if base current is the input current and collector current is the output current. Generally, the quantity of greatest interest to the circuit designer is voltage amplification. Figure 10.3 shows a simple configuration of a BJT voltage amplifier. This circuit is known as the common emitter configuration.

A voltage source is not typically used to forward bias the base–emitter junction in an actual circuit, but we will assume that VBB is used for this purpose. A value of VBB or VBE near 0.6–0.7 V would be appropriate for this situation. The collector supply would be a larger voltage such as 12 V. We will assume that the value of VBB sets the DC emitter current to a value of 1 mA for this circuit. The collector current entering the BJT will be slightly <1 mA, but we will ignore this difference and assume that IC = 1 mA also. With a 4-kW collector resistance, a 4-V drop will appear across RC leading to a DC output voltage of 8 V. The distribution of electrons across the base region for the steady-state or quiescent conditions is shown by the solid line in Figure 10.3.

If a small AC voltage now appears in series with VBB, with a peak value much less than VBB, the injected electron density on the left side of the base region will be modulated. Since this density varies exponentially with the applied voltage (see Eq. [10.2]), a small AC voltage can cause considerable changes in density. The broken lines in Figure 10.3 show the distributions at the positive and negative peak voltages. The collector current may change from its quiescent level of 1 mA to a maximum of

1.1 mA as vin reaches its positive peak and to a minimum of 0.9 mA when vin reaches its negative peak. The output collector voltage will drop to a minimum value of 7.6 V as the collector current peaks at 1.1 mA and will reach a maximum voltage of 8.4 V as the collector current drops to 0.9 mA. The peak- to-peak AC output voltage is then 0.8 V. The peak-to-peak value of vin to cause this change might be 5 mV, giving a voltage gain of A = -0.8/0.005 = -160. The negative sign occurs because when vin increases, the collector current increases, but the collector voltage decreases. This represents a phase inversion in the amplifier of Figure 10.3.

In summary, a small change in base-to-emitter voltage causes a large change in emitter current. This current is channeled across the collector and through the load resistance, across which a larger incremental voltage develops. The ratio of incremental output voltage to incremental input voltage is called the amplification factor or, more commonly, the voltage gain of the circuit.

Bipolar Junction Transistor Circuits-0007

The Digital Switch

Figure 10.4 demonstrates a simple switching circuit that functions as a logic level inverter. The two logic levels representing binary 1 and 0 might be +5 and 0 V, respectively. If the lower level is applied to the input, that is, vin = 0 V, the base–emitter junction is not forward-biased and the BJT is in its cutoff region. Since no collector current flows through the resistance, no voltage is dropped across this element and the collector or output voltage equals the power supply voltage of 5 V. If the input voltage is raised to 5 V, the base current increases to

Bipolar Junction Transistor Circuits-0008

where VBEon is typically 0.7 V. The collector current increases toward b IB causing the voltage drop across the load resistance to increase. The collector voltage drops to VCEsat, perhaps 0.2 V, and limits further increase in IC. This circuit behaves as an inverter. A low input voltage results in a high output voltage and vice-versa. This type of circuit forms the basis of many logic circuits and is discussed further in a later section.

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