Transistor

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Transistor
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BJT Transistors
NPN Transistor
Sandwiching a P-type layer between two n-type
layers.
                                               
                            
PNP Transistor
Sandwiching a N-type layer between two p-type
layers.
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How a NPN Transistor works?
                                         
The base-emitter diode (forward) acts as a
switch. when v1gt0.7 it lets the electrons flow
toward collector. so we can control our output
current (Ic) with the input current (Ib) by using
transistors.
E
C
B
Forward
backward
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Transistors have three terminals
Collector
Base
Emitter
Active Always on IcBIb
Saturation IcIsaturation On as a switch
Transistors work in 3 regions
Off Ic0 Off as a switch
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Transistor as a Switch

• Transistors can be used as switches.1

Transistor
Switch

• Transistors can either
• conduct or not conduct current.2

• ie, transistors can either be on or off.2

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Transistor Switching Example15
X
12V
Variable Voltage Supply

• When VBE is less than 0.7V the transistor is off
• and the lamp does not light.

• When VBE is greater than 0.7V the transistor is
  on
• and the lamp lights.

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Amplifier example
As you see, the transistor is biased to be always
on. The input signal is amplified by this
circuit. The frequency of output is the same as
its input, but the polarity of the signal is
inverted.
The measure of amplification is the gain of
transistor.
Example Input Amplitude 0.2v Output
amplitude10v Gain10/0.250
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Field Effect Transistors
JFET
MOSFET
CMOS
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How a JFET transistor works?
When the gate is negative ,it repels the electron
in the N-channel. So there is no way for
electrons to flow from source to drain.
When the negative voltage is removed from Gate
,the electrons can flow freely from source to
drain .so the transistor is on.
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How a MOSFET Transistor works?
In MosFET, the Gate is insulated from p-channel
or n-channel. This prevents gate current from
flowing, reducing power usage.
When the Gate is positive voltage ,it allows
electrons to flow from drain to source .In this
case transistor is on.
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How a CMOS transistor works?
N-channel P-channel MOSFETs can be combined in
pairs with a common gate .
When Gate (input) is high ,electrons can flow in
N-channel easily . So output becomes low.
(opposite of input)
When Gate (input) is low ,holes can flow in
P-channel easily. So output becomes high.
(opposite of input)
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Opamp
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Schematic diagram of lm741
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Ideal Opamp
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Operational Amplifier (OP AMP)

• Basic and most common circuit building device.
  Ideally,
• No current can enter terminals V or V-. Called
  infinite input impedance.
• VoutA(V - V-) with A ?8
• In a circuit V is forced equal to V-. This is
  the virtual ground property
• An opamp needs two voltages to power it Vcc and
  -Vee. These are called the rails.

A
Vo (A V -A V ) A (V - V )

-

-
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INPUT IMPEDANCE
WHY? For an instrument the ZIN should be very
high (ideally infinity) so it does not divert any
current from the input to itself even if the
input has very high resistance. e.g. an opamp
taking input from a microelectrode.
Input Circuit Output
Impedance between input terminals input
impedance
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OUTPUT IMPEDANCE
WHY? For an instrument the ZOUT should be very
low (ideally zero) so it can supply output even
to very low resistive loads and not expend most
of it on itself. e.g. a power opamp driving a
motor
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OPAMP COMPARATOR
VoutA(Vin Vref) If VingtVref, Vout 8 but
practically hits ve power supply Vcc If
VinltVref, Vout -8 but practically hits ve
power supply -Vee
A (gain) very high
Application detection of QRS complex in ECG
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OPAMP ANALYSIS

• The key to op amp analysis is simple
• No current can enter op amp input terminals.
• gt Because of infinite input impedance
• The ve and ve (non-inverting and inverting)
  inputs are forced to be at the same potential.
• gt Because of infinite open loop gain
• These property is called virtual ground
• Use the ideal op amp property in all your analyses

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OPAMP VOLTAGE FOLLOWER
V VIN. By virtual ground, V- V Thus Vout
V- V VIN !!!!
So whats the point ? The point is, due to the
infinite input impedance of an op amp, no current
at all can be drawn from the circuit before VIN.
Thus this part is effectively isolated. Very
useful for interfacing to high impedance sensors
such as microelectrode, microphone
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OPAMP INVERTING AMPLIFIER

1. V- V
2. As V 0, V- 0
3. As no current can enter V- and from Kirchoffs
   Ist law, I1I2.

4. I1 (VIN - V-)/R1 VIN/R1 5. I2 (0 -
VOUT)/R2 -VOUT/R2 gt VOUT -I2R2 6. From 3 and
6, VOUT -I2R2 -I1R2 -VINR2/R1 7. Therefore
VOUT (-R2/R1)VIN
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OPAMP NON INVERTING AMPLIFIER

1. V- V
2. As V VIN, V- VIN
3. As no current can enter V- and from Kirchoffs
   Ist law, I1I2.

4. I1 VIN/R1 5. I2 (VOUT - VIN)/R2 gt VOUT
VIN I2R2 6. VOUT I1R1 I2R2 (R1R2)I1
(R1R2)VIN/R1 7. Therefore VOUT (1 R2/R1)VIN
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SUMMING AMPLIFIER
Recall inverting amplifier and If I1
I2 In
If
VOUT -Rf (V1/R1 V2/R2 Vn/Rn)
Summing amplifier is a good example of analog
circuits serving as analog computing amplifiers
(analog computers)! Note analog circuits can
add, subtract, multiply/divide (using logarithmic
components, differentiate and integrate in real
time and continuously.
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DRIVING OPAMPS

• For certain applications (e.g. driving a motor or
  a speaker), the amplifier needs to supply high
  current. Opamps cant handle this so we modify
  them thus

Irrespective of the opamp circuit, the small
current it sources can switch ON the BJT giving
orders of magnitude higher current in the load.