1. Semiconductor circuit elements and dependent sources
Lecture C
Semiconductor circuit elements and dependent sources

2. Semiconductor devices and active circuit elements
Examples:
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)
Bipolar Junction Transistors (BJTs)
Operational amplifiers (op-amps)

3. Active circuit elements - MOSFETs
MOSFETs can be modeled as dependent sources
MOSFETs can be n-channel (n-FETs) or p-channel (p-FETs)
Construction and operation of nFETS:

4. MOSFET operation – continued
Application of a voltage difference between the gate and source allows current to flow from the drain to the source.

5. N-channel MOSFETs Description of behavior: Notes:
Increasing vGS increases drain current
Notes:
External power supply required!
Low power requirements at gate
ID = 0 unless vGS exceeds the threshold voltage (VT)
MOSFETs have 3 terminals
Gate, source, drain
Circuit symbol:

6. Fluid system analogy General MOSFET behavior: Valve in fluid system
Applied gate-to-source voltage allows current to flow from drain to source
Valve in fluid system

7. Example MOSFET circuit
If vGS < VT  ID = 0 and vOUT = vDS; increasing vGS reduces vOUT

8. MOSFETs as switches
If the gate voltage of the MOSFET toggles between two values, the MOSFET can behave as a switch:

9. MOSFETs as dependent sources
Simple saturation-region MOSFET model:
Notes:
VT is the threshold voltage; ID = 0 if vGS < VT
Approximate i-v curves:

10. Analog and digital signals
Signals (voltages and currents) can be thought of as being either analog or digital
Analog signals are continuous
(they take on all intermediate
values when they change)
Digital signals are discrete
(they can change abruptly
between values)

11. Reminder: annotate previous slide to show analog, digital signal.

12. Analog and digital signals – continued
Whether a signal (or circuit) is treated is being analog or digital is dependent upon how you want to model the circuit (it depends on the application)
For example, our MOSFET can act in two ways:
A MOSFET acting as a switch is generally treated as a digital circuit element
A MOSFET acting as a dependent source is generally treated as an analog circuit element

13. p-channel MOSFETs
Operation of p-channel MOSFETs (p-FETs) is “similar” to n-FETs, EXCEPT:
Negative gate voltage applied, and current direction reversed
Operation is “inverted” from the n-FET operation

14. Demos:
Show MOSFETS
Show variation in source current with gate voltage. (Emphasize that external power supply is required)

15. Active circuit elements - BJTs
BJTs – Bipolar Junction Transistors
BJTs can be npn or pnp
npn BJT construction:
BJTs can also be used as dependent power sources
BJTs provide a current controlled current source: the base current is used to control the collector-to-emitter current

16. Discussion:
Probably counter-productive to try to do a detailed discussion of BJT operation.
What (basically) happens is that an applied base current allows a much larger current to flow from the collector to the emitter

17. npn BJTs – continued BJTs have three terminals:
Base, collector, emitter
npn BJT circuit symbol:
Approximate i-v curves:

18. Note that the emitter current is equal to the collector current plus the base current (it is approximately equal to the collector current, since the base current is generally small)

19. BJTs as dependent sources
Simple active-region BJT model:
 is the current gain; it is generally large (often over 100)
BJTs are commonly used as amplifiers. Low power at the base is converted to high power at the collector/emitter.

20. Demo:
Show BJTs
Illustrate change in emitter current with base current (voltage). Emphasize external power supply.

21. Active circuit elements - operational amplifiers
Operational amplifier (or op-amp) based circuits are often used to perform mathematical operations
Operational amplifiers are constructed of a number of transistors, but are typically represented by the circuit symbol:

22. Annotate previous slide to show:
Two inputs
One output
External power supplies (it’s an active circuit element)

23. Notes about operational amplifiers
Op-amp circuits have two inputs and one output
Op-amps require (generally) two external power supply inputs
There is (ideally) no current flow into the input terminals
The op-amp absorbs no power from the circuit
The output voltage is the difference in the input voltages, multiplied by a large number (ideally, infinity)
However, the output voltage cannot exceed the range set by the external power supplies

24. Example op-amp circuits
Inverting voltage amplifier:
This overall circuit can be modeled as a dependent source (it is a voltage controlled voltage source)

25. Example op-amp circuits
Differencing circuit:
This circuit can also be modeled as a voltage controlled voltage source (except two voltages control the output)

26. Demo:
Show op-amp
Show op-amp circuits; emphasize external power supplies. Mention voltage rails limit output voltages.