Inserting a resistor RE in the emitter circuit as in Figure below causes degeneration, also known as negative feedback. This opposes a change in emitter current IE due to temperature changes, resistor tolerances, beta variation, or power supply tolerance. Typical tolerances are as follows: resistor— 5%, beta— 100-300, power supply— 5%. Why might the emitter resistor stabilize a change in current? The polarity of the voltage drop across RE is due to the collector battery VCC. The end of the resistor closest to the (-) battery terminal is (-), the end closest to the (+) terminal it (+). Note that the (-) end of RE is connected via VBB battery and RB to the base. Any increase in current flow through RE will increase the magnitude of negative voltage applied to the base circuit, decreasing the base current, decreasing the emitter current. This decreasing emitter current partially compensates the original increase.
Note that base-bias battery VBB is used instead of VCC to bias the base in Figure above. Later we will show that the emitter-bias is more effective with a lower base bias battery. Meanwhile, we write the KVL equation for the loop through the base-emitter circuit, paying attention to the polarities on the components. We substitute IB≅IE/β and solve for emitter current IE. This equation can be solved for RB , equation: RB emitter-bias, Figure above.
Before applying the equations: RB emitter-bias and IE emitter-bias, Figure above, we need to choose values for RC and RE . RC is related to the collector supply VCC and the desired collector current IC which we assume is approximately the emitter current IE. Normally the bias point for VC is set to half of VCC. Though, it could be set higher to compensate for the voltage drop across the emitter resistor RE. The collector current is whatever we require or choose. It could range from micro-Amps to Amps depending on the application and transistor rating. We choose IC = 1mA, typical of a small-signal transistor circuit. We calculate a value for RC and choose a close standard value. An emitter resistor which is 10-50% of the collector load resistor usually works well.
Our first example sets the base-bias supply to high at VBB = VCC = 10V to show why a lower voltage is desirable. Determine the required value of base-bias resistor RB. Choose a standard value resistor. Calculate the emitter current for β=100 and β=300. Compare the stabilization of the current to prior bias circuits.
An 883k resistor was calculated for RB, an 870k chosen. At β=100, IE is 1.01mA.
For β=300 the emitter currents are shown in Table below.
Emitter current comparison for β=100, β=300.
| Bias circuit | IC β=100 | IC β=300 |
|---|---|---|
| base-bias | 1.02mA | 3.07mA |
| collector feedback bias | 0.989mA | 1.48mA |
| emitter-bias, VBB=10V | 1.01mA | 2.76mA |
Table above shows that for VBB = 10V, emitter-bias does not do a very good job of stabilizing the emitter current. The emitter-bias example is better than the previous base-bias example, but, not by much. The key to effective emitter bias is lowering the base supply VBB nearer to the amount of emitter bias.
How much emitter bias do we Have? Rounding, that is emitter current times emitter resistor: IERE = (1mA)(470) = 0.47V. In addition, we need to overcome the VBE = 0.7V. Thus, we need a VBB >(0.47 + 0.7)V or >1.17V. If emitter current deviates, this number will change compared with the fixed base supply VBB,causing a correction to base current IB and emitter current IE. A good value for VB >1.17V is 2V.
The calculated base resistor of 83k is much lower than the previous 883k. We choose 82k from the list of standard values. The emitter currents with the 82k RB for β=100 and β=300 are:
Comparing the emitter currents for emitter-bias with VBB = 2V at β=100 and β=300 to the previous bias circuit examples in Table below, we see considerable improvement at 1.75mA, though, not as good as the 1.48mA of collector feedback.
Emitter current comparison for β=100, β=300.
| Bias circuit | IC β=100 | IC β=300 |
|---|---|---|
| base-bias | 1.02mA | 3.07mA |
| collector feedback bias | 0.989mA | 1.48mA |
| emitter-bias, VBB=10V | 1.01mA | 2.76mA |
| emitter-bias, VBB=2V | 1.01mA | 1.75mA |
How can we improve the performance of emitter-bias? Either increase the emitter resistor RE or decrease the base-bias supply VBB or both. As an example, we double the emitter resistor to the nearest standard value of 910Ω.
The calculated RB = 39k is a standard value resistor. No need to recalculate IE for β = 100. For β = 300, it is:
The performance of the emitter-bias circuit with a 910 emitter resistor is much improved. See Table below.
Emitter current comparison for β=100, β=300.
| Bias circuit | IC β=100 | IC β=300 |
|---|---|---|
| base-bias | 1.02mA | 3.07mA |
| collector feedback bias | 0.989mA | 1.48mA |
| emitter-bias, VBB=10V | 1.01mA | 2.76mA |
| emitter-bias, VBB=2V, RE=470 | 1.01mA | 1.75mA |
| emitter-bias, VBB=2V, RE=910 | 1.00mA | 1.25mA |
As an exercise, rework the emitter-bias example with the emitter resistor reverted back to 470Ω, and the base-bias supply reduced to 1.5V.
The 33k base resistor is a standard value, emitter current at β = 100 is OK. The emitter current at β = 300 is:
Table below below compares the exercise results 1mA and 1.38mA to the previous examples.
Emitter current comparison for β=100, β=300.
| Bias circuit | IC β=100 | IC β=300 |
|---|---|---|
| base-bias | 1.02mA | 3.07mA |
| collector feedback bias | 0.989mA | 1.48mA |
| emitter-bias, VBB=10V | 1.01mA | 2.76mA |
| emitter-bias, VBB=2V, RB=470 | 1.01mA | 1.75mA |
| emitter-bias, VBB=2V, RB=910 | 1.00mA | 1.25mA |
| emitter-bias, VBB=1.5V, RB=470 | 1.00mA | 1.38mA |
The emitter-bias equations have been repeated in Figure below with the internal emitter resistance included for better accuracy. The internal emitter resistance is the resistance in the emitter circuit contained within the transistor package. This internal resistance rEE is significant when the (external) emitter resistor RE is small, or even zero. The value of internal resistance REE is a function of emitter current IE, Table below.
Derivation of rEE
rEE = KT/IEm where: K=1.38×10-23 watt-sec/oC, Boltzman's constant T= temperature in Kelvins ≅300. IE = emitter current m = varies from 1 to 2 for Silicon rEE ≅ 0.026V/IE = 26mV/IE
For reference the 26mV approximation is listed as equation rEE in Figure below.
Emitter-bias equations with internal emitter resistance rEE included..
The more accurate emitter-bias equations in Figure above may be derived by writing a KVL equation. Alternatively, start with equations IE emitter-bias and RB emitter-bias in Figure previous, substituting RE with rEE+RE. The result is equations IE EB and RB EB, respectively in Figure above.
Redo the RB calculation in the previous example emitter-bias with the inclusion of rEE and compare the results.
The inclusion of rEE in the calculation results in a lower value of the base resistor RB a shown in Table below. It falls below the standard value 82k resistor instead of above it.
Effect of inclusion of rEE on calculated RB
| rEE? | rEE Value |
|---|---|
| Without rEE | 83k |
| With rEE | 80.4k |
Bypass Capacitor for RE
One problem with emitter bias is that a considerable part of the output signal is dropped across the emitter resistor RE (Figure below). This voltage drop across the emitter resistor is in series with the base and of opposite polarity compared with the input signal. (This is similar to a common collector configuration having <1 gain.) This degeneration severely reduces the gain from base to collector. The solution for AC signal amplifiers is to bypass the emitter resistor with a capacitor. This restores the AC gain since the capacitor is a short for AC signals. The DC emitter current still experiences degeneration in the emitter resistor, thus, stabilizing the DC current.
What value should the bypass capacitor be? That depends on the lowest frequency to be amplified. For radio frequencies Cbpass would be small. For an audio amplifier extending down to 20Hz it will be large. A “rule of thumb” for the bypass capacitor is that the reactance should be 1/10 of the emitter resistance or less. The capacitor should be designed to accommodate the lowest frequency being amplified. The capacitor for an audio amplifier covering 20Hz to 20kHz would be:
Note that the internal emitter resistance rEE is not bypassed by the bypass capacitor.
Article extracted from Tony Kuphaldt’s Lesson in Electric circuits Volume III Semiconductors under the terms and conditions of Design Science License.