Transistor Flashcards
What is a transistor?
a semiconductor device with three connections, capable of amplification in addition to rectification.
Which way does current flow in a NPN transistor?
Collector, base, emitter
Which way does current flow in a PNP transistor?
Emitter, base, Collector
If the base current is 0, what is the collector current?
Always 0 as the collector current is controlled by the base current
What does Vt (thermal voltage) equal?
kT/q
What does the collector current equal?
Ic= Is*e^(Vbe/Vt) Ic = beta Ib
Which part of the graph shows the active region?
The parts with the steepest slope, between saturation voltages (graph of x=Vbe against y=Vce)
How does a BJT amplifier work?
By using the base-emitter voltage to control the collector current.
Only possible in the active region otherwise it’s saturated
What happens to Vce when there is a small change in Vbe?
Large change
What does Vce equal?
Vce = Vs - IcR
What is gm?
The mutual conductance
What does gm equal?
gm = Ic/Vt
What is the small signal ratio between Vc and Ic?
Vc = -Rc * ic
= -(Ic/Vt)*Rc
What is the gain of a simple common-emitter amplifier?
A = - Rc * gm
Why is the gain unpredictable?
related to beta which has a very wide tolerance (50-200)
When does the transistor become saturated?
When Vce = 0 as it can’t go any lower and the collector current cannot go any higher
What are the common assumptions for quiescent conditions?
- DC implies that capacitors look like open circuits
- Vbe = 0.5-0.7
- Transistor current gain is high so emitter and collector currents are equal, base is negligible
What does input impedance equal?
rin = Rb // Beta/ gm
What does Rc always equal?
r out
What do coupling capacitors do?
limit the range of frequency for which it operates
What is the cut off frequency?
- 3dB point
- when the gain of the amplifier falls by a factor of root 2
What does the cut off frequency equal?
fc = 1 / 2pirin*cin
What should C out be greater than for the lower cut off frequency?
C out»_space; 1/ 2pifc/10 *rout
What does re equal?
re = Vt/Ic
What should Ce be greater than?
Ce >= 1/ 2pifc/10*re
For large signal analysis what do Ic1 and Ic2 equal?
Ic1 = Is*e^(vbe1/vt)
What is the input impedance for a differential amplifier?
rin = 2 beta/gm
Why are input and output coupling capacitors used?
to remove dc bias voltages
What are the non ideal dc effects?
-removes dc base base current and a small dc base bias current is required to get into the active region
When are FET amplifiers used?
- required input impedance is very high
- load current requirements are high
When are BJT amplifiers used?
- high voltage gain is needed
- input and output impedances are of the same order
How do you calculate percentage distortion
A/4Vt
What are the problems with the CE amplifier?
- fro inputs bigger then around 10 mV the output is heavily distorted
- only works for ac signal
- often a differential input is needed
Compare CE with differential
CE -one transistor -single input, single output -high gain possibly with high input impedance DIF -need at least two transistors -differential input and out put -reduced gain possible with high input impedance
What are the single stage amplifier limitations?
- high gain = high gm and high Rc
- high run = low gm
- low r out = low rc
How do you increase the gain?
use two amplifiers instead of one
Compare transistors and Op amps?
OP -simple to use -near ideal properties TRANS -better frequency response -lower noise
What is the voltage swing of a differential amplifier?
Ic = gm Vt
Vout = Vin - RcIc
V swing = Vin - Vout
What is the overall gain of a two stage amplifier?
ADiff * Ace* rce/(rdiff + rce)
