BJT Flashcards
How does a bipolar transistor work? Include description of DC bias voltages needed.
NPN or PNP: both amplify input signals.
If the base-emitter voltage (VBE) is increased from 0V to ≈0.7V, then this turns the base-emitter diode on. This causes electron current to flow from the emitter across the base-emitter junction. However, the base layer is very thin (i.e. the diffusion distance of the electrons in the p-type region) so most of the electrons, instead of exiting through the base terminal are propelled across the reverse biased collector junction and exit through the emitter terminal.
What is the cutoff mode?
The BJT is fully off in this state. In the cutoff mode both the base emitter as well as collector base junction is reverse biased. The BJT is equivalent to an open switch in this mode.
What is the saturation mode?
The transistor is fully on in this state. The CB as well as BE junctions are forward biased. The BJT operates like a closed switch in the saturation mode. If a BJT is in saturation mode than it should satisfy the following condition,
|IC| = β.|IB|
Where β is common emitter current amplification factor or current gain.
What is the Active Mode?
In order to use the transistor as an amplifier, it must be operated in the active mode. The BE junction is forward biased whereas the CB junction is reverse biased. Figure attached shows both n-p-n and p-n-p transistors biased in the active mode of operation.
Name the three main BJT configurations:
- Common Emitter,
- Common Base,
- Common Collector (Emitter Follower)
What is one application of the Common Collector circuit:
The low output impedance allows a source with a large output impedance to drive a small load impedance; it functions as a voltage buffer. In other words, the circuit has current gain instead of voltage gain.
A small change to the input current results in a much larger change in the output current supplied to the output load.
What is one application of the Common Emitter Circuit?
Low frequency voltage amplifier:
The input capacitor C removes any constant component of the input, and the resistors R1 and R2 bias the transistor so that it will remain in active mode for the entire range of the input.
The output is an inverted copy of the AC-component of the input that has been amplified by the ratio (RC/RE) and shifted by an amount determined by all four resistors.
Because RC is often large, the output impedance of this circuit can be prohibitively high. To alleviate this problem, RC is kept as low as possible and the amplifier is followed by a voltage buffer like an emitter follower.
What is one Application of the Common Base configuration?
Current buffer or voltage amplifier.
In this circuit the emitter terminal of the transistor serves as the input, the collector the output, and the base is connected to ground, or “common”, hence its name.
What is the Operating Bias Point/Q point?
Operating bias point is the point on the DC load line (a load line is the graph of output current vs. output voltage in any of the transistor configurations) which represents the DC current through the transistor and voltage across it when no ac signal is applied. The Q point represents the DC biasing condition.
What is the name and purpose of a BJT with Q point halfway between cutoff and saturation?
CLASS-A amplifier: The purpose of CLASS-A bias is to make the amplifier relatively free from distortion by keeping the signal waveform out of the region between 0V and about 0.6V where the transistor’s input characteristic is non linear.
Class A design produces good linear amplifiers, but are wasteful of power as a standing bias current flows during the whole waveform cycle, even when no signal is being amplified.
Name the three common types of biasing configuration:
- Fixed Bias/Base Bias
- Collector to Base Bias
- Self Bias/Voltage Divider Bias (most common)
How can we imagine the Early Effect in circuit terms?
A high valued resistance in parallel with the constant current source
When performing DC analysis of a BJT, list appropriate steps to take:
- Open Circuit all Capacitors
- Short Circuit all AC Voltage sources
- Separate bias circuit and make output (base) open
- Make equivalent Thevenin circuit of bias circuit
- Plug in DC model for BJT (Diode and ßIB)
How do we calculate Thevenin Resistance?
Remove the power sources : voltage sources => wires, current sources => breaks.
Remove load.
Calculate total resistance between open connection points.
N.B. For bias circuit, we regard BJT base as our load, ‘looking back’.
How do we calculate the Thevenin Voltage?
Remove the load from the original circuit
Calculate voltage across the open connection points.
Low Pass Filter Transfer Function?
G(ω)= 1/(1+jωRC)
