What is Emitter Bias Circuit
Emitter bias is a very good and stable way to bias transistors if both positive and negative power supplies are available. Emitter bias fluctuates very little with temperature variation and transistor replacement.
Fig. 9.15 shows the emitter bias circuit. This circuit differs from base-bias circuit in two important respects. First, it uses two separate d.c. voltage sources ; one positive (+ VCC) and the other negative (– VEE). Normally, the two supply voltages will be equal. For example, if VCC = + 20V (d.c.), then VEE = – 20V (d.c.). Secondly, there is a resistor RE in the emitter circuit.
We shall first redraw the circuit in Fig. 9.15 as it usually appears on schematic diagrams. This means deleting the battery symbols as shown in Fig. 9.16. All the information is still (See Fig. 9.16) on the diagram except that it is in condensed form. That is a negative supply voltage – VEE is applied to the bottom of RE and a positive voltage of + VCC to the top of RC
Circuit Analysis of Emitter Bias
Fig. 9.16 shows the emitter bias circuit. We shall find the Q-point values (i.e. d.c. IC and d.c. VCE) for this circuit.
(i) Collector current (IC). Applying Kirchhoff’s voltage law to the base-emitter circuit in Fig. 9.16, we have,
It is clear that IC is dependent on VBE and β, both of which change with temperature. If Re >>Rb /β, then expression for IC becomes :
This condition makes IC (~ IE ) independent of VBE. If IC (~ IE ) is independent of β and VBE, the Q point is not affected appreciably by the variations in these parameters. Thus emitter bias can provide stable Q-point if properly designed.
Example 9.12. For the emitter bias circuit shown in Fig. 9.18, find IE, IC,VC and VCE for β = 85 and VBE = 0.7V.
Example 9.13. Determine how much the Q-point in Fig. 9.18 (above) will change over a temperature range where β increases from 85 to 100 and VBE decreases from 0.7V to 0.6V.
Biasing with Collector Feedback Resistor
In this method, one end of RB is connected to the base and the other end to the collector as shown in Fig. 9.19. Here, the required zero signal base current is determined not by VCC but by the collector base voltage VCB. It is clear that VCB forward biases the base-emitter junction and hence base current IB flows through RB. This causes the zero signal collector current to flow in the circuit. Circuit analysis. The required value of RB needed to give the zero signal current IC can be determined as follows. Referring to Fig. 9.19,
It can be shown mathematically that stability factor S for this method of biasing is less than (β + 1) i.e. Stability factor, S < (β + 1) Therefore, this method provides better thermal stability than the fixed bias.
Note. It can be easily proved (See **example 9.17) that Q-point values (IC and VCE) for the circuit shown in Fig. 9.19 are given by ;
Advantages of Emitter bias
- It is a simple method as it requires only one resistance RB .
- (ii) This circuit provides some stabilisation of the operating point as discussed below : VCE = VBE + VCB
Suppose the temperature increases. This will increase collector leakage current and hence the total collector current. But as soon as collector current increases, VCE decreases due to greater drop across RC. The result is that VCB decreases i.e. lesser voltage is available across RB. Hence the base current IB decreases. The smaller IB tends to decrease the collector current to original value.
Disadvantages Emitter bias
- The circuit does not provide good stabilisation because stability factor is fairly high, though it is lesser than that of fixed bias. Therefore, the operating point does change, although to lesser extent, due to temperature variations and other effects.
- This circuit provides a negative feedback which reduces the gain of the amplifier as explained hereafter. During the positive half-cycle of the signal, the collector current increases. The increased collector current would result in greater voltage drop across RC. This will reduce the base current and hence collector current.
Example 9.14. Fig. 9.20 shows a silicon transistor biased by collector feedback resistor method.
Determine the operating point. Given that β = 100.
- It is required to set the operating point by biasing with collector feedback resistor at IC = 1mA, VCE = 8V. If β = 100, VCC = 12V, VBE = 0.3V, how will you do it ?
- What will be the new operating point if β = 50, all other circuit values remaining the same ?
Solution. VCC = 12V, VCE = 8V, IC = 1mA β = 100, VBE = 0.3V
(i) To obtain the required operating point, we should find the value of RB.
Now, collector load is
Comments. It may be seen that operating point is changed when a new transistor with lesser β is
used. Therefore, biasing with collector feedback resistor does not provide very good stabilisation. It
may be noted, however, that change in operating point is less than that of base resistor method.
Example 9.16. It is desired to set the operating point at 2V, 1mA by biasing a silicon transistor with collector feedback resistor RB. If β = 100, find the value of RB.
Example 9.17. Find the Q-point values (IC and VCE) for the collector feedback bias circuit shown in Fig. 9.22.
Solution. Fig. 9.22 shows the currents in the three resistors (RC, RB and RE) in the circuit. By following the path through VCC, RC, RB, VBE and RE and applying Kirchhoff’s voltage law, we have,
VCC – (IC+IB) RC– IB RB– VBE – IERE = 0
Example 9.18. Find the d.c. bias values for the collector-feedback biasing circuit shown in Fig 9.23. How does the circuit maintain a stable Q point against temperature variations ?
Stability of Q-point.
We know that β varies directly with temperature and VBE varies inversely with temperature. As the temperature goes up, β goes up and VBE goes down. The increase in β increases IC (= βIB). The decrease in VBE increases IB which in turn increases IC. As IC tries to increase, the voltage drop across RC (= ICRC) also tries to increases. This tends to reduce collector voltage VC (See Fig. 9.23) and, therefore, the voltage across RB. The reduced voltage across RB reduces IB and offsets the attempted increase in IC and attempted decrease in VC . The result is that the collector feedback circuit maintains a stable Q-point. The reverse action occurs when the temperature decreases.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta