Transistors and Amplifiers Flashcards
NPN type
bipolar junction transistors
operate when the base is positive
PNP
bipolar junction transistors
operate when the base is negative
All transistors will operate when they reach
the SATURATION VOLTAGE which is 0.7V
the saturation voltage
0.7V
Transistors have two functions:
They can act as an electronic switch when the saturation voltage is achieved
They can amplify current or voltage
The current gain of the transistor is the amount by which the base current is amplified and is known by Hғᴇ
Hғᴇ = Ic/Ib
Hғᴇ = Current Gain (no unit) Ic = Collector Current (A) Ib = Base Current (A)
If asked to find weather the transistor is on or not that means calculate
Vʙᴇ
If the transistor is on assume that
Vʙᴇ = 0.7V unless stated otherwise
Voltage Amplification
A transistor can also be used to amplify voltage this can be achieved by adding a load resistor to the collector
Av = Vo/Vi
Av = Voltage Gain (no unit) Vo = Output Voltage (V) Vi = Input Voltage (V)
ONLY in voltage amplification does
Ic = Ie as current is negligible
How to protect a transistor
Putting a diode across a relay as it protects the transistor from back EMF.
If you want to drive a large load using the same power supply you must use a
Darlington Pair
Hғᴇ = Hғᴇ₁ x Hғᴇ₂
Hғᴇ = Current Gain
Darlington Pair saturation voltage is
1.4V
Hғᴇ =
these values are interchangeable
= Av
these values are interchangeable
MOSFET
Metal Oxide Semiconductor Field Effect Transistor
MOSFET notation
G = Gate S = Source D = Drain
MOSFETs are used to
amplify current based on the voltage
this means the size of the input current will vary according to the size of the input, voltage threshold, voltage.
The basic operation of a MOSFET
When the voltage on the gate is below gate threshold, there is in effect no channel, no current flow source drain
Above the gate threshold voltage a channel is produced connecting the drain and source, producing a drain current
The greater the gate voltage, the better the device conducts the bigger the drain current
At saturation the MOSFET preforms at its best, lowest source/drain resistance, highest drain current.
Increasing Vɢs
Increases Id
When the MOSFET saturates it can’t get any more
When Vsᴀᴛ is reached Id cannot increase anymore
Id = Id (on)
Vds = Vgs- Vt
Vds = Voltage Drain Source (V) Vgs = Voltage Gate Source (V) Vt = Thermistor Voltage (V)
If the MOSFET is saturated
Vgs = Vsᴀᴛ
Calculating Transconductance
gₘ = ΔId / ΔVgs
gₘ = transconductance (AV-1) ΔId = Change in Id (A) ΔVgs = Change in Vgs (V)
The Push - Pull Driver
If Vɪɴ is +0.7 (positive) then the NPN will switch on and flow anticlockwise.
If Vɪɴ is -0.7V (negative) then the PNP will switch on and flow clockwise.
The Comparator
Two input signals and one output
the output cannot be greater than 85%.
The op amp compares two separate voltage signals
The Difference Amplifier
both inputs are used and the op amp amplifies the difference between the two signals.
if there is no difference the output = 0V
A difference amplifier is used to produce proportional control.
The Inverting Amplifier
The signal is inverted, positive to negative.
The Non-Inverting Amplifier
It is used to multiply the input voltage by the gain, but in this case it does not invert the signal.
The Summing Amplifier
Combines the voltage of two or more inputs into one single output.
The Voltage-Follower
An op amp configuration that ALWAYS has the voltage gain of 1. It does not provide any amplification to the signal.
