Analysis of Circuits Flashcards
Define p-type and n-type Silicon
P-type Silicon is doped with Group 3 atoms. These atoms readily accept electrons due to their higher concentration of holes.
N-type Silicon is doped with Group 5 atoms. These atoms have one extra free electrons to be used for conduction. So N-type has free electrons for conduction at room temperature.
Describe the features of the p-n junction
The p-n junction is formed when p-type and n-type silicon recombine. The free electrons from the n-type combine with the holes of the p-type creating a depletion region.
When the p-type is positive (forward bias) the electrons are repelled from the negative terminal and the depletion regions narrows.
When the p-type is negative (reverse bias) the carriers are attracted to the positive terminal causing the depletion region to widen.
What forms the basis of the Diode
P-n junction
Describe the construction of a Bi-Polar transistor
There are two types of BJT - pnp and npn. In each case a BJT is formed by putting two diodes back to back.
Why is the BJT said to be bi-polar
The name bipolar arises as both charge carriers and holes take part in the current flow. In npn carriers dominate the flow whereas in pnp holes dominate the flow
What are the relative levels of doping in a pnp transistor
The emitter is n-type and highly doped
The collected is n-type and the least doped
What is the basic principle underlying BJT operation
Due to the thin base and high doping of the emitter, a small change in the base voltage (base current) leads to a large change in the collector current.
What is the current gain
The current gain, hfe is the ratio of the collector to base current - Ic/Ib
What is the minimum voltage needed for the BJT to function
VBE ~ 0.7V
What are the criteria for choosing an BJT operating point
- Stay wishing the safe limits
- Ensure the max value of Ic is not exceeded
- Choose the highest value of Vcc possible
- Set Vce to be as close to Vcc/2 as possible. This will give the maximum voltage swing at the output.
How can the collector resistance be found
By applying nodal voltage analysis we can draw a load line Ic = (Vcc - Vce)/Rc
Define the small signal parameters
hfe - current gain
hre - reverse voltage transfer ratio
hie - input resistance
hoe - output admittance
What are the key characteristics of the Basic Common Emitter
The basic common emitter has a base resistance to fix the operating point and no emitter resistance (gain not stabilised). The common emitter has a large negative gain and so is used as a single stage inverting amplifier
What are the key characteristics of a stabilised common emitter circuit
The stabilised common emitter has a potential divider to fix the base voltage. It also has an emitter resistance to stabilise the gain. However the emitter resistance reduces the gain and so often a bypass capacitor is used (shorts the resistor in the SSM)
What is the key assumption when analysing the stabilised common emitter
If the current gain is v.large/infinite then we can assume the base current is negligible. Therefore we can use potential divider to find the input voltage. If the current gain is not infinite we must draw the a Thevenin equivalent circuit.
VT = (R2/R1+R2) * Vcc RT = R1*R2/R1+R2
What is the benefit of adding a resistor between the base and the collector
The resistor acts as a negative feedback loop. This has all the benefits of negative feedback (e.g stabilised gain, increased input resistance etc.)