CHAPTER 5
Transistor
Circuits
OBJECTIVES
Describe and Analyze:
• Need for bias stability
• Common Emitter Amplifier Biasing
• RC-coupled Multistage Amplifiers
• Direct-Coupled Stages
• Troubleshooting
Introduction
The DC bias values for VCE and Ic are collectively
called the “Q-Point”. Because a transistor’s beta
varies 2 to 1 or more from device to device, biasing
circuitry needs to be designed so that the Q-point is
not a function of beta.
Likewise, the gain of a transistor amplifier should not
depend on beta. Gain should be set by the values of
external components such as resistors.
Beta Changes with
Temperature
Not only does it vary from device to device,
beta is also strongly dependent on
temperature.
Voltage Divider Biasing
• Choose Rb1 & Rb2 so that: Rb1 || Rb2 << Re
for the worst-case value of beta
• Vb is fixed by Rb1 and Rb2, and: Ve = Vb – 0.7V
• Re >> r’e. Therefore Ic = Ie = Ve / Re
Biasing Example
For a circuit like the one on the previous slide, calculate Vb,
Ve, Ie, Ic, Vc, and Vce given:  = 50
Vcc =12V, Rb1 = 100k, Rb2 = 20k, Rc = 4k, Re = 2k,
Vb = [Rb2 / (Rb1 + Rb2)]  Vcc = 12V / 6 = 2 Volts
Ve = Vb – 0.7 = 2 – 0.7 = 1.3
Ic = Ie = Ve / Re = 1.3V / 2k = 0.65 mA
Vc = Vcc - Rc  Ic = 12V – 4k  1.3mA = 6.8V
Vce = Vc – Ve = 6.8V – 1.3V = 5.5V
r’e = 25mV / Ie = 25mV / 0.65mA = 38.5 Ohms
Is Re >> r’e? Is 2000 >> 38.5 ? Yes!
Input Impedance
Zin will not depend on  if: Rb1 || Rb2 << Re
Voltage Gain: Unbypassed Re
Av = rc / Re where rc = Rc || RL Gain is stable but low
Voltage Gain: Bypassed Re
Av = rc / r’e where rc = Rc || RL. But r’e = 25mV / Ie
Gain is high, but changes with the signal current
Voltage Gain: Compromise
A trade-off between high gain and gain stability
Emitter Biasing
Very stable Q-point, but requires two voltage supplies
Emitter Bias Example
For a circuit like that of the previous slide, calculate
Ie, Ic, Ve, Vc, Vce given
Vcc = +12V, Vee = -12V, RE + Re = 10k, Rc = 4.7k
Since, effectively, Vb is zero, Ve = -0.7V
Ie = (Ve – Vee) / Re =11.3V / 10k = 1.13mA
Ic is about the same as Ie, so Ic = 1.13mA
Vc = Vcc – Rc  Ic = 12V – 4.7k  1.13mA = 6.7V
Vce = Vc – Ve = 6.7V – 0.7V = 6.0 Volts
Voltage-Mode Feedback
Can never saturate or cut off. High gain. Limited Vce.
RC-Coupled Stages
Circuit is no longer used, but illustrates the principle.
Choosing Capacitors
Key Idea:
Compared to the values of Zin and Zout, the
reactances of the capacitors (Xc) should be negligible
in the frequency range the input signals.
• Xc = 1 / (2fC)
• Xc << Zin and Xc << Zout
Xc Compared with Zin or Zout
What ratio of Z to Xc is required to say that Xc is negligible
compared to Zin or Zout? Not as high as you might assume.
Zin and Zout are determined by resistors. Let Zx be the sum of
Xc and R. But remember, it’s a vector (phasor) sum: Zx = sqrt[
R2 + X2 ]
Let Xc be about a third of R. That is, Xc = .3R
Then Zx = sqrt[ R2 + .09R2 ] = R  sqrt(1.09) = 1.04R
So there is only a 4% effect if Xc is as big as a third of Zin or
Zout.
A Numerical Example
The first stage of a two-stage amplifier has an output
impedance of 2k. The input impedance of the second stage is
4k. The frequency range is 50 Hz to 5000 Hz. Select a
coupling capacitor.
Since Zout < Zin, we will compare Xc to Zout to be
conservative.
Let Xc = .3  Zout = .3  2k = 600 Ohms.
Xc is highest at the low end of the frequency range.
Xc = 1 / 2f C => C = 1 / 2f Xc
C = 1 / 6.28  50  600 = 5.3 uF
A 10 uF electrolytic capacitor should do nicely.
Direct Coupled Amplifiers
Having PNP as well as NPN transistors allows us to do
away with coupling capacitors
Gain of a Multi-Stage Amp
Suppose you have two single-stage amplifiers, each
with a voltage gain of 20. If the stages are coupled
together, will the gain be 20  20 = 400?
Not necessarily. In fact, probably not!
The problem is that Zin of stage two “loads down” the
output of stage one. With a transistor amp, the Zin of
the second stage is effectively in parallel with the Rc of
the first stage. So the voltage gain (Av) will be:
Av = (Rc || Zin) / Re
Troubleshooting
• Check the power-supplies, but keep your fingers off
any high-voltage that may be present.
• Check the DC bias levels with no signal applied.
• Check for shorted capacitors.
• Check for open capacitors.
• Try signal tracing using amplifier’s “normal” input.
• Try signal tracing with an injected signal.
• Try disconnecting one stage from the next, but
remember to use resistors to simulate Zout.