Lecture-10

ELECTRONIC CIRCUITS

LESSON 10 INTRODUCTION TO BIASING OF TRANSISTOR CIRCUIT Objective In this lesson, I will be explaining you the concept of biasing done in the transistor circuit. The biasing of transistor is done to fix the operating point or Q-point of the transistor in Active region, Saturation region and Cut-off region of operation. I will explain you a simple biasing circuit in this lesson. The input and output characteristics will also be discussed with you. At the end of the lesson we will talk about the design and mathematical aspect of the biasing.

Lesson My guide to biasing differs from conventional approach, in that I have started by describing the effects that biasing has on the output waveform, before moving on to the bias circuits. A bipolar junction transistor, (BJT) is very versatile. It can be used in many ways, as an amplifier, a switch or an oscillator and many other uses too. Before an input signal is applied its operating conditions need to be set. This is achieved with a suitable bias circuit, some of which I will describe. A bias circuit allows the operating conditions of a transistor to be defined, so that it will operate over a pre-determined range. This is normally achieved by applying a small fixed dc voltage to the input terminals of a transistor. Bias design can take a mathematical approach or can be simplified using transistor characteristic curves. The characteristic curves predict the performance of a BJT. There are three curves, an input characteristic curve, a transfer characteristic curve and an output characteristic curve. Of these curves, the most useful for amplifier design is the output characteristics curve. The output characteristic curves for a BJT are a graph displaying the output voltages and currents for different input currents. The linear (straight) part of the curve needs is utilized for an amplifier or oscillator. For use as a switch, a transistor is biased at the extremities of the graph, these conditions are known as “cutoff” and”saturation”.

After the initial bend, the curves approximate a straight line. The slope or gradient of each line represents the output impedance, for a particular input base current. So what has all this got to do with biasing? Take, for example the middle curve. The collector emitter voltage is displayed up to 20 volts. Let’s assume that we have a single stage amplifier, working in common emitter mode, and the supply voltage is 10 volts. The output terminal is the collector, the input is the base, where do you set the bias conditions? The answer is anywhere on the flat part of the graph. However, imagine the bias is set so that the collector voltage is 2 volts. What happens if the output signal is 4 volts peak to peak ? Depending on whether the transistor used is a PNP or NPN, then one half cycle will be amplified cleanly, the other cycle will approach the limits of the power supply and will “clip”. This is shown below :

Output Characteristics Curves

For each transistor configuration, common emitter, common base and emitter follower the output curves are slightly different. A typical output characteristic for a BJT in common emitter mode are shown below :-

The above diagram shows a 4 volt peak to peak waveform with clipping on the positive half cycle. This is caused by setting the bias at a value other than half the supply voltage.

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is a BC107A. The values of Rb and Rc can be determined by either mathematical approach or by using the output characteristic curves for the BC107A.

Quiescent Point (Q-Point)

The lower diagram shows the same amplifier, but here the bias is set so that collector voltage is half the value of the supply voltage. Hence, it is a good idea to set the bias for a single stage amplifier to half the supply voltage, as this allows maximum output voltage swing in both directions of an output waveform. Input Characteristic Curves-

Before describing the bias circuits, it is worthwhile looking at a typical input characteristic curve for a small signal BJT. The following is the input characteristic for a transistor in common emitter mode, it is a plot of input base emitter voltage verses base current. It is shown with both x and y axis slightly zoomed.

The point Vo in the diagram above is where the output signal would be taken. For simplicity, the input signal and coupling capacitors have been omitted. For minimum distortion and clipping it is desirable to bias this point to half the supply voltage, 10 volts dc in this case. This is also known as the quiescent point. The ac output signal would then be superimposed on the dc bias voltage. The Q-point is sometimes indicated on the output characteristics curves for a transistor amplifier. The quiescent point also refers to the dc conditions (bias conditions) of a circuit without an input signal. Q-Point Value

I have mentioned that setting the Q-point to half the supply voltage is a good idea. It gives a circuit the highest margin for overload. However, any amplifier will clip if the input amplitude exceeds the limit for which the circuit was designed. However, there are certain cases when it is not necessary to bias a stage to half the supply voltage. Examples would be an RF amplifier design where the input signal is in microvolts or millivolts. If the stage had a gain of 200 then the output (assuming a 2mV peak input) would only need to swing up and down 400mV about the Q-point. Hence a stage with a supply voltage of 12 volts could have its Q-point set at 10 volts or even 2 volts without problems. Another example would be a microphone stage where similar low level input signals are involved. Output Characteristic Curve for a BC107A

The base emitter voltage, Vbe is quoted in most text books as either 0.6 V or 0.7 V Both values are an approximation, and as can be seen from the above graph the value of Vbe varies between this range. For small signal work with base currents of 50uA or below a value for Vbe of 0.6 volts is a reasonable quote. For higher base currents, a Vbe of 0.7 V is a better approximation. In fact, in a large power transistor, the Vbe value can be even higher. The value of Vbe also varies widely with temperature change. Simple Bias Circuit

The simplest bias circuit is shown below. It consists only of a fixed bias resistor and load resistor. The BJT is operating in common emitter mode. The dc current gain or beta, h FE is the ratio of dc collector current divided by dc base current. The BJT 32

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Bias Design

The collector voltage Vc for the simple bias design is 10 volt. The dc current gain, h FE for the BC107A is obtained from the manufacturer’s data sheets and varies between devices. A typical beta is around 290. Taking a base current of 20uA and reading values direct from the output curves, the collector current, for a collector emitter voltage of 10 volts is around 3.9mA. As h FE= Ic / Ib then a BC107A must have a beta of at least 3.9mA / 20uA = 195 to work with this circuit. Also, the base emitter voltage, Vbe is typically 0.6v. Knowing the above data and using ohm’s law, values for Rb and Rc can be determined: Rb = Vcc - Vbe / Ib = (20-0.6) / 20u = 970k use (1M) Rc = Vc / Ic = 10 / 3.9m = 2.56K use (2.7K) Mathematical ApproachWithout using the output characteristic curve, values for Rb and Rc can still be calculated. A value for h FE must be estimated first and a desired collector current. As h FE varies in each transistor the value chosen should be the lowest value from the manufacturer’s data sheets. The equations to use are: Rc = Vc / Ic Ib = Ic / h FE Rb = Vcc - 0.6 / Ib. Using the example above with Vcc=20 and h FE =195 yields the same values. Conclusion

In this lesson we have studied the basic concepts of biasing done in a transistor circuit. The need of biasing was discussed with you. I think now you would have idea about why the biasing is been done in the transistor. Then we discussed the input and output characteristics of the transistor and how to fix the operating or Q-point of the transistor operation. We studied a simple biasing circuit in this lesson with design and mathematical approach.

Notes

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