1. Electronics Fundamentals
Circuits, Devices, and Applications
Floyd
Chapter 17

2. Summary Bipolar junction transistors (BJTs)
The BJT is a transistor with three regions and two pn junctions. The regions are named the emitter, the base, and the collector and each is connected to a lead.
There are two types of BJTs – npn and pnp.
Separating the regions are two junctions.
C (Collector) C
Base-Collector junction n p
B (Base) p B n n
Base-Emitter junction p
E (Emitter) E

3. Summary
BJT biasing
For normal operation, the base-emitter junction is forward-biased and the base collector junction is reverse-biased.
For the pnp transistor, this condition requires that the base is more negative than the emitter and the collector is more negative than the base.
For the npn transistor, this condition requires that the base is more positive than the emitter and the collector is more positive than the base.
BC reverse- biased + +
pnp
npn +
BE forward- biased +

4. Summary
BJT currents
A small base current (IB) is able to control a larger collector current (IC). Some important current relationships for a BJT are: IC I IB I IE I

5. Summary Voltage-divider bias
Because the base current is small, the approximation
is useful for calculating the base voltage.
After calculating VB, you can find VE by subtracting 0.7 V for VBE. R1 RC VC
Next, calculate IE by applying Ohm’s law to RE: VB VE R2 RE
Then apply the approximation
Finally, you can find the collector voltage from

6. Summary Example: Solution: Voltage-divider bias
Calculate VB, VE, and VC for the circuit.
Solution:
3.02 V
+15 V
VE = VB V =
2.32 V R1 RC
27 kW
2.2 kW
2N3904 R2 RE
6.8 kW
1.0 kW
9.90 V

7. Summary
CE amplifier
In a common-emitter amplifier, the input signal is applied to the base and the output is taken from the collector. The signal is larger but inverted at the output.
VCC
Output coupling capacitor RC R1 C2
Input coupling capacitor C1 R2
Bypass capacitor C3 RE

8. Summary
CC amplifier
In a common-collector amplifier, the input signal is applied to the base and the output is taken from the emitter. There is no voltage gain, but there is power gain.
The output voltage is nearly the same as the input; there is no phase reversal as in the CE amplifier.
VCC R1 C1
The input resistance is larger than in the equivalent CE amplifier because the emitter resistor is not bypassed. R2 RE

9. Summary The BJT as a switch
BJTs are used in switching applications when it is necessary to provide current drive to a load.
VCC
VCC
In switching applications, the transistor is either in cutoff or in saturation. RC RC
In cutoff, the input voltage is too small to forward-bias the transistor. The output (collector) voltage will be equal to VCC.
VOUT
IIN = 0
= VCC
IIN > IC(sat)/bDC
= 0 V
When IIN is sufficient to saturate the transistor, the transistor acts like a closed switch. The output is near 0 V.

10. Selected Key Terms Bipolar junction transistor (BJT) Class A amplifier
Saturation
A transistor with three doped semiconductor regions separated by two pn junctions.
An amplifier that conducts for the entire input cycle and produces an output signal that is a replica of the input signal in terms of its waveshape.
The state of a transistor in which the output current is maximum and further increases of the input variable have no effect on the output.

11. Selected Key Terms Cutoff Q-point Amplification Common-emitter (CE)
Class B amplifier
The non-conducting state of a transistor.
The dc operating (bias) point of an amplifier.
The process of producing a larger voltage, current or power using a smaller input signal as a pattern.
A BJT amplifier configuration in which the emitter is the common terminal.
An amplifier that conducts for half the input cycle.

12. Selected Key Terms Junction field-effect transistor (JFET)
MOSFET
Depletion mode
Enhancement mode
A type of FET that operates with a reverse-biased junction to control current in a channel.
Metal-oxide semiconductor field-effect transistor.
The condition in a FET when the channel is depleted of majority carriers.
The condition in a FET when the channel has an abundance of majority carriers.

13. Selected Key Terms Common-source
Oscillator
Feedback
An FET amplifier configuration in which the source is the common terminal.
A circuit that produces a repetitive waveform on its output with only a dc supply voltage as an input.
The process of returning a portion of a circuit’s output signal to the input in such a way as to create certain specified operating conditions.