TYPES OF IDEAL AMPLIFIER
Outline:
• Introduction to Amplifier
• Block Diagram of Audio Amplifier
• Ideal Voltage Amplifier
• Ideal Current Amplifier
• Ideal Trans resistance Amplifier
• Ideal Trans conductance Amplifier
AMPLIFIER
An amplifier is a electronic device which increase the power level of week
electrical signal without changing its shape.
An amplifier is an electronic device which amplifies (increases) the amplitude or power
of a signal without changing signal shape.
An electronic circuit which increases the strength of weak electrical
signal
without changing its shape.
IN OUT
GAIN
V A V ,I
,I
CONT..
Block diagram of Amplifier Ideal Amplifier Model
GAIN (A): (measure of amplification)
• It is the ratio of output quantity divided by input quantity.
• Shows the amplified difference between the input and output.
• It has no unit.
CONT..
There are three types of amplifier gain:
1. Voltage Amplifier Gain (𝐴𝑉)
2. Current Amplifier Gain (𝐴𝐼)
3. Power Amplifier Gain (𝐴𝑃)
Voltage Gain: 𝐴v 𝑂𝑢𝑡𝑝𝑢𝑡
=
𝐼𝑛𝑝𝑢𝑡
𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝑉𝑜𝑙𝑡𝑎𝑔𝑒
Current Gain: 𝐴i 𝑂𝑢𝑡𝑝𝑢𝑡
=
𝐼𝑛𝑝𝑢𝑡
𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝐶𝑢𝑟𝑟𝑒𝑛𝑡
𝑂𝑢𝑡𝑝𝑢𝑡
= 𝐼𝑛𝑝𝑢𝑡
𝐴p
Power Gain:
𝑃𝑜𝑤𝑒𝑟
𝑃𝑜𝑤𝑒𝑟
.
Calculation of gain in Decibels (dB):
Voltage Gain: 𝐴𝑉 (dB) = 20 𝑙𝑜𝑔 𝐴𝑉
Current Gain: 𝐴𝐼 ( 𝑑B ) = 20 𝑙𝑜𝑔 𝐴𝐼
Power Gain: 𝐴𝑃 ( 𝑑𝐵) = 10 𝑙𝑜𝑔 𝐴𝑃
Basics of logarithm and antilogarithm
Gain = +3dB indicates that output signal is doubled and Gain = -3dB indicates that output signal is halved (Loss).
Characteristics for an ideal amplifier:
Gain (A) should remain constant for different values of input signal.
It should be constant for input signal having different frequencies.
It must not add noise to the output signal.
It should not be affected by temperature changes.
It must remain stable over long periods of time.
CONT..
Example: Find the Gain in dB in the following case-
(a) Voltage gain of 30
(b) power gain of 100
Solution: (a) Given, voltage gain (Av) =30
As we know that, voltage gain in dB = 20log (Av) = 20log (Vo/Vin)
= 20log30 = 20 x 1.477
= 29.54 dB
(b) Given, Power gain (Ap) = 100
Power gain in dB = 10log (Ap) =10 log (Po/Pin)
= 10log 100
= 10 x 2 = 20 dB
.
Example: Determine voltage, current and power gain of an amplifier having an input signal of 1mA at
10mV and a corresponding output signal of 10mA at 1V. Express all gains in decibels (dB).
Block diagram of an Audio Amplifier:
VOLTAGE AMPLIFIER
Voltage Amplifier intend to amplify input voltage signal and provide the output voltage signal .
It is voltage dependent voltage source (VCVS)
𝑉𝑠𝑅𝑖
Input ckt: (Thevnin’s Equivalent)
𝑅𝑠 + 𝑅𝑖
Vi=
• If, 𝑅𝑖 ≫ 𝑅𝑠 𝑡ℎ𝑒𝑛, 𝑉𝑖 ≈ 𝑉𝑠
Av Vi 𝑅𝐿
Output ckt: (Thevnin’s Equivalent)
𝑉o =
Voltage controlled voltage source
𝑅𝑜 + 𝑅𝐿
• If, 𝑅𝑜 ≪ 𝑅𝐿 𝑡ℎ𝑒𝑛, 𝑉𝑜 ≈ 𝐴𝑉𝑉𝑖
• VS = Source Voltage, RS = Source Resistance
𝑽𝒐 = 𝑨𝑽𝑽𝒔 → 𝑽𝒐 ∝ 𝑽𝒔
• Ri = Input Resistance, Ro = Output Resistance
• Vi = Input Voltage, Vo = Output Voltage
Hence for an ideal voltage amplifier, Ri= ∞ and Ro =0
• Ii = Input Current, Io = Output Current
𝑨v= Open circuit voltage gain =Vo/Vi=𝒘𝒉𝒆𝒏 𝑅𝐿 ∞
• RL = Load Resistance
**This amplifier provides a voltage output proportional to the voltage input and the proportionality factor is
independent of magnitude of source and load resistance.
CURRENT AMPLIFIER
Current Amplifier intend to amplify input current signal and provide the output current signal
It is current dependent current source
𝐼𝑠
Input ckt: (Norton’s
𝐼
= 𝑅𝑅
Equivalent)
𝑠 𝑆
• If, 𝑅𝑖 ≪ 𝑅𝑠 𝑡ℎ𝑒𝑛, 𝐼𝑖+≈𝑅𝐼𝑠𝑖
Output ckt: (Norton’s
𝐴𝑖 𝐼𝑖
𝐼 =
Equivalent)
𝑅
𝑅𝑜𝑜
• If, 𝑅𝑜 ≫ 𝑅𝐿 𝑡ℎ𝑒𝑛, 𝐼+ 𝑅
𝑜≈ 𝐿
𝐴𝑖 𝐼𝑖
• IS = Source Current, RS = Source Resistance
𝑨𝑰 = 𝑨circuit
𝒊 𝑰𝒔 → 𝑰𝒐 𝒘𝒉𝒆𝒏 𝑹
• Ri = Input Resistance, Ro = Output Resistance
=𝒐short
∝ 𝑰𝒔
current gain • Vi = Input Voltage, Vo = Output Voltage
Hence, for an ideal current amplifier, 𝑹𝒊 ==𝟎𝟎𝒂𝒏𝒅 𝑹𝒐
=Io/Ii
• Ii = Input Current,
=∞
Io = Output Current
• RL = Load Resistance
**This amplifier provides an output current proportional to the input current and the proportionality
factor is independent of magnitude of source and load resistance.
THANK YOU
