2P5 Electrical Engineering Flashcards
Where do electrons and holes go in a BJT?
electrons travel from emmitter to collector,
holes travel from base to emmittter
What is hFE in a BJT?
IC/IB
What are the performance limits for the BJT?
The linear limit (before saturation region)
Max Ic
Maximum power (I=1/V)
Maximum Vce
What is hFE for a small signal model?
Current gain at constant VCE
๐๐ผC/๐๐ผB
What is hoe
๐Ic/๐VCE
What is hie?
dVBE/dIB
What is hre?
dVBE/dVce (usually neglible)
How does the small signal circuit change for pnp?
works the same, but things are reversed
How can you make a common emmitter amplifier more stable?
Take R1 from the collector
What is the advantage of adding emitter resistance?
Decreases the gain, but increases the stability of the DC operating point. But placing a capacitor in parallel can give large midband gain.
Why is the instability of the operating point bad?
hFE can vary a lot (between 100 and 500), so VCE is very sensitive
How does the emitter follower work?
It is a buffer, collector directly connected to Vcc, leads to unity gain
What happens with a common mode signal in a differential amplifier?
Source voltages both rise the same amount, reducing gate source voltage, therefore small output
How can common mode gain be model?
Split into two parallel circuits, with 2R3 in each one
How can differential mode gain be modelled?
As no R3
How does current mirror work?
Two BJTs, one with the collector connected to the base
What is the gain with negative feedback with gain A and return ratio B?
gain = A/(1+AB)
What is AB refferred to in negative feedback?
Loop gain.
How does gain vary with a change in A?
dG/G = dA/A (1/(1+AB))
robust in variations in A
How is bandwidth affected by feedback?
Lowers the 3dB frequency by (1+AB)
What is the nature of gain bandwidth product?
Stays constant.
How does negative feedback afffect input impedance and output impedance?
Rout decreases by a factor of 1+AB
Rin increases by a factor of 1+AB
WHatโs the difference to an op amp whether it uses BJTs or FETS for the input differential pair?
FET has high Rin
BJT has high A and is stable, needs a constant current source.
What are the rules for ideal op amps?
Output attempts to do whatever is necessary to make voltage difference between the inputs equal to zero
The inputs draw no current
What are input bias currents for op amps?
When input pair is a BJT differential amplifier, input bias currents are needed. For the 741, the currents are around 100nA. Leads to inaccuracy in outputs
What is the input offset voltage for a non-ideal op amp?
The voltage needed between inverting and non-inverting to drive output to zero.
What methods to solve non-ideal op amp?
Sum currents at the inverting input.
How does analogue adder work?
input resistances, then inverting amplifier.
How can you make an integrator with an op amp?
Using a capacitor in parallel with the op amp on an inverting amplifier set up
How can you make a differentiator with an op amp?
Capacitor before the op amp, and resistor in parallel with op amp.
What is a risk with the transimpedance amplifier?
Large R good for large amplification, however, parasitic C will reduce high freq response
What is a gyrator?
A component that acts like an inductor but can fit on a silicon chip.
Features 2 resistors and capacitor, inverting input connected to outpu
What is the maximum efficiency of a class A amplifier and how is this derived?
25%, itโs derived from the AC power out/DC power in, which can be found from
Pout = vo^2/(2Rl)
Pin = VccIco
where Ico is the current flowing through the load.
What does the transfer characteristic of a class B amplifier look like?
Deadband (0V output) for -0.7V<V<0.7V, then linear outside this range until it reaches Vcc or -Vc
How does a class B amplifier works?
When v1>0.7V, T1 is turned on and therefore conducts and begins to behave as an emitter follower. Reverse happens for v1<-0.7V with T2.
T1 is npn (normal)
but T2 is pnp
How does a class AB amplifier work?
placing diodes, which bias T1 and T2 base voltages to 0.7V and -0.7V respetively, this reduces the crossover distortion.
They need current sources (or mirrors) in order to hold the diodes at said voltages.
Conditions for oscillations?
AB = -1, therefore infinite gain and oscillations will occur
