Transistors and mosfets
What is the minimum base resistor used for a small junction transistor
1K Ὡ Resistor
What is the typical input resistance for a MOSFET
1MὩ Resistor
What is the maximum current that can pass into the base of a small junction transistor
5mA
P-channel MOSFET
A MOSFET which amplifies a negative current.
N-channel MOSFET
A MOSFET which amplifies a positive current.
What is a transistor
A transistor can either act as a driver or a power switch. This device amplifies currents to up to 100A and voltages up to 1000V.
What are the advantages and disadvantages of a MOSFET compared to a junction transistor
Advantages: very large input resistance very large current gain It has a positive thermal coefficient, if its temperature increases the resistance from drain to source increases and so decreases the drain current flowing. Disadvantages: more expensive than a junction transistor
Explain and show the graph of a n-channel MOSFET is arranged as a switch and also show the circuit diagram
As shown on the graph the output voltage does not change much until Vin is greater than 1V ( Voltage from the gate to the source). Drain current passes and the MOSFET saturates at about 0.1V when Vin is about 2V. Any further increase in Vin has no effect on Vout. The MOSFET is therefore acting as a switch in the same way as the transistor.
What are the three terminals of a transistor
Base Collector Emitter
What are the three terminals of a MOSFET
Gate Drain Source
What is a MOSFET
It is a driver or power amplifier. They are voltage operated three terminal devices.
When selecting a MOSFET, what should all of them be able to do
Operate at the power supply. Pass sufficient drain current. Dissipate sufficient power. Give a very low drain to source resistance.
When selecting a transistor what should they all be able to do
Operate at the supply voltage. Pass sufficient collector current. Dissipate sufficient power. Give a large current gain.
How does a transistor work
The base-emitter junction of a transistor behaves like a forward biased diode. When a small current passes through the base emitter junction a much larger current is made to pass through the collector and emitter. Since the base emitter behaves like a forward biased diode, for a silicon diode there will be a voltage of 0.7V between the base and emitter when a collector current passes.
Show and explain the graph that shows how Vout varies with Vin for a npn transistor arranged as a switch and what is the circuit diagram
The graph shows that the output voltage remains at the supply voltage until Vin reaches 0.5V This is because, at this voltage a small collector current flows and Vout begins to fall. When Vin reaches about 0.7V, the maximum collector current is flowing (limited by R1) and the transistor saturates with a collector emitter voltage of 0.2V. Any further increase in Vin will have no effect on Vout. When Vin is less than 0.7V, Vout is equal to the supply voltage. When Vin is greater than 0.7V, Vout is 0.2V.
What is a pnp transistor
This is a transistor which amplifies a negative current.
What is a npn transistor
This is a transistor which amplifies a positive current.