Topological Classification of Amplifiers:
Discuss types of amplifiers based on input and output parameters or topological classification of amplifiers
Voltage amplifiers
Current amplifiers
Transresistance amplifiers
Transconductance amplifiers
How do we select input and output impedance of various amplifier topologies
Comparison of ideal and real amplifiers
All amplifiers can be categorised according to the input and output parameters. This is the basic classification. This classification is based on the users’ requirements. The output signal can be voltage or current and similarly, a voltage signal can either be current or voltage.
Voltage amplifier
Current amplifier
Transconductance amplifier
Transconductance amplifier
Voltage Amplifier:
The voltage signal is applied at the input and gets the amplified voltage signal at the output. This is voltage amplification. The ratio of output voltage to the input voltage is termed voltage gain Av. It is the internal gain of the amplifier. At any instant, Vout is proportional to Vin.
\[A_V = \frac {V_{out}}{V_{in}}\]
Design Guide: The input impedance of voltage amplifiers should be high (ideally infinite) and output impedance should be very low (ideally zero).
Let me explain why??
Consider an ideal system. Av is independent of source and load resistor.
A voltage source with zero internal resistance. And input impedance is set to infinite. At this condition, the whole voltage signal without any loss appears on the output circuit.
\[R_i = \infty\]
\[R_S = 0 \]
\[V_i = V_S\]
At the output circuit, output resistance Ro, should be zero, so there's no voltage drop across it. The voltage signal is amplified by the factor Av appears at the load.
\[R_O = 0\]
\[R_L = \infty\]
\[V_O = A_VV_S\]
Now consider a real system. Source resistance can not be zero, and input impedance can not be infinite. To minimize signal loss, input impedance should be high as compared to source resistance.
\[R_i \rightarrow \text{high as compared to } R_S\]
\[R_S = low\]
\[V_i = \frac {V_SR_i}{R_i+R_S}\]
At the output circuit, output resistance Ro, can not be zero. To minimize signal loss, Ro should be very less compared to load resistance.
\[R_O = low\]
\[R_L \rightarrow \text {high as compared to } R_O\]
\[V_O = \frac {A_V*V_i*R_L}{R_L+R_O}\]
Current Amplifier:
The current signal is applied at the input and the amplified current is the signal at the output. This is current amplification. The ratio of output current to the input current is termed as current gain AI. AI is the internal current gain. At any instant Iout is proportional to Iin.
\[A_i = \frac {I_{out}}{I_{in}}\]
Design Guide: The input impedance of current amplifiers should be low (ideally zero) and output impedance should be very high (ideally infinite).
Let me explain why.
Consider an ideal system. Ai should be independent of load resistor and source resistor.
A current source with infinite internal resistance. And input impedance is set to zero. At this condition, the whole current signal without any loss appears on the output circuit. See figure below.
\[R_i=0\]
\[R_S = \infty \]
\[I_i = I_S\]
At the output circuit, output resistance Ro, should be infinite, so there's no current flowing across it. The current signal is amplified by the factor Ai appearing at the load.
\[R_L = 0\]
\[R_O = \infty\]
\[I_L = A_i*I_S\]
Now consider a real system. Source resistance can not be infinite, and input impedance can not be zero. To minimize signal loss, input impedance should be very low as compared to source resistance.
\[R_i = low\]
\[R_S \rightarrow \text {high as compared to } R_i\]
\[I_i = I_S(\frac{R_S}{R_S+R_i})\]
At the output circuit, output resistance Ro, can not be infinite. To minimize signal loss, Ro should be very high compared to the load resistance.
\[R_L = low \]
\[R_O \rightarrow \text {high as compared to }R_L\]
\[I_L = A_i*I_i(\frac {R_O}{R_O+R_L})\]
Transresistance Amplifier:
The current signal is applied at the input and gets the amplified voltage signal at the output. The ratio of output voltage to the input current is termed transfer gain. At any instant, Vout is proportional to Iin.
\[R_M = \frac {V_{out}}{I_{in}}\]
Design Guide: The input and output impedance of transresistance amplifiers should be low (ideally zero).
Consider an ideal system. RM should be independent of source and load resistor. In transresistance amplifiers input is a current source. The internal resistance of the source is infinite, and the input impedance is zero. There is no signal drop across the zero impedance path.
\[R_i = 0\]
\[R_S = \infty\]
\[I_i = I_S\]
At the output side, we get voltage amplified by factor RM. The internal resistance of the voltage source is zero. So we get a rated output signal, that is voltage.
\[R_O = 0\]
\[R_L = \infty\]
\[V_O = R_M*I_S\]
Consider the real system. The internal resistance of the current source can not be infinite and input resistance can be zero. So, choose your current source such that it has very high internal resistance as compared to the input impedance.
\[R_i = low\]
\[R_S \rightarrow \text {high as compared to } R_i\]
\[I_i = I_S(\frac {R_S}{R_S+R_i})\]
At the output circuit, the internal resistance of the voltage source can not be zero. Choose a load which has high resistance as compared to the output impedance.
\[R_O = low\]
\[R_L\rightarrow \text {high as compared to }R_O \]
\[V_O = R_M*I_i(\frac {R_L}{R_L+R_O})\]
Transconductance Amplifier:
The voltage signal is applied at the input and get the amplified current signal at the output. The ratio of output current to the input voltage is termed transfer gain. At any instant Iout is proportional to Vin.
\[G_M = \frac{I_{out}}{V_{in}}\]
Design Guide: The input and output impedance of transconductance amplifiers should be high (ideally infinite).
Consider an ideal system. GM should be independent of source and load resistor.
In transconductance amplifiers input is a voltage source. The internal resistance of the source is zero, and the input impedance is infinite. There is no voltage drop, the whole signal appears at the output circuit.
\[R_i = \infty \]
\[R_S = 0\]
\[V_i = V_S\]
At the output side, we get current amplified by factor GM. The output resistance Ro is infinite. Ro is in parallel with the load resistor, the small load resistance allows maximum current to flow through it. The whole current signal appears on the load resistor.
\[R_L = 0\]
\[R_O = \infty\]
\[I_L = G_M*V_S\]
Consider the real system. In transconductance amplifiers input is a voltage source. The voltage source has some internal resistance, and the input impedance can't be infinite. Input impedance should be higher than source resistance. At this condition, you can get the maximum possible voltage at the output circuit.
\[R_i \rightarrow \text {high as compared to }R_S\]
\[R_S = low\]
\[V_i = V_S(\frac{R_i}{R_i+R_S})\]
At the output side, we get the current signal amplified by factor GM . Output resistance can't be infinite. Select output impedance such that it is higher enough than the load resistor. Since Ro is in parallel with the load resistor, small load resistance allows maximum current to flow through it.
\[R_L = low\]
\[R_O \rightarrow \text {high as compared to } R_L\]
\[I_L = G_M*V_i (\frac {R_O}{R_O+R_L})\]
At the end:
This is not the end, it is just an introductory article on amplifiers. There is a wide range of amplifiers available, there are so many classifications. Like operational amplifiers, low noise amplifiers, feedback amplifiers, audio, video and radio amplifiers etc.
Reference:
ANALOG ELECTRONICS
By A. KANDASWAMY, ANDRÉ PITTETUnderstand Amplifiers by Owen Bishop
Electronic Devices and Circuits
By Cheruku, Cheruku Dharma Raj
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