 Hello everyone. In this session, we are going to learn about classification of basic amplifiers. The learning outcome of this session, at the end of this session, students will be able to describe basic types of amplifiers. These are the contents of today's session. So we are going for amplifier definitions, the voltage amplifier. We can define a voltage amplifier is an amplifier which amplifies mostly the voltage level of input signal and provides output voltage and current signal that is the voltage amplifier. So the voltage amplifier mainly amplifies the current voltage level of input signal, low level input signal at the output and the current amplifier can be defined as the amplifier which amplifies mostly the current level of the input signal and provides a proportional output voltage and current signal. So that is the current amplifier. Now basically the power amplifier can be defined as it is a combination of voltage amplifier and current amplifier. The power amplifier is an amplifier which amplifies both the voltage level as well as current level of input signal, low level input signal and provides both output amplified output voltage as well as amplified output current signal. So input is low level AC signal and output is amplified power signal. Now let us go for the classification of amplifier. Amplifiers are classified into four broad categories. So these are categorized as a voltage amplifier, current amplifier, transconductance amplifier and transresistance amplifier. So the deciding factors for category categorization of this amplifier is that the relative magnitude of the input impedance or relative value of the input impedance of amplifier in comparison with the input signal source impedance and the output impedance of amplifier in comparison with the output load resistance impedance. So considering input impedance and source impedance and output impedance of amplifier and output load resistance or output load impedance, the amplifiers are basically classified into four broad categories. So first category that is first type of amplifier that is a voltage amplifier. So the figure shows this figure shows the equivalent circuit for a voltage amplifier in which this amplifier is designed to amplify a voltage signal input voltage signal. The input is a voltage signal and the output is amplified a voltage signal. So this voltage amplifier is just like a voltage controlled a voltage source voltage controlled voltage source. So this amplifier in this amplifier it is required to high input impedance and low output impedance. For this voltage amplifier it is required to be the input impedance should be high and output impedance should be low. So for voltage amplifier input signal is a voltage signal again the output signal is amplified a voltage signal. So the ideal voltage amplifier provides a amplified output voltage that is proportional to the input voltage signal and the proportionality constant the proportionality constant called as a transfer ratio or gain. So that is called as a voltage gain is a ratio of amplified output voltage to the input low level input voltage. So the transfer ratio forward transfer ratio or gain of this voltage amplifier that is called as a voltage gain. So that is designated as AV it is independent of it is independent of input source impedance and output load impedance. For ideal voltage amplifier for ideal voltage amplifier the input impedance should be infinite and output impedance of amplifier should be 0. So this in this equivalent circuit the input signal source voltage is V s its internal resistance is shown in series with this voltage source. R n is a input resistance of voltage amplifier and R out is the output resistance of a voltage amplifier. R L is the load connected at the output of voltage amplifier. So V out stands for output voltage appearing across load resistor R L. So effective input voltage for amplifier is a gain. So from this figure of equivalent circuit for a voltage amplifier it is seen that if input resistance of this voltage amplifier that is R in is much less than the source input signal source impedance R s in that case the effective input voltage of this amplifier gain is approximately equal to input signal source voltage V s. So if load resistance connected at the output of this amplifier is much greater than the output resistance of amplifier or output impedance of amplifier. So in that case the amplified output voltage V out so that is approximately equal to AV. AV stands for the forward transfer ratio or gain of this amplifier. So that is a voltage gain AV multiplied by the input voltage input voltage of this amplifier. So AV stands for a voltage gain of this amplifier voltage amplifier. So it is a ratio of amplified output voltage to the input voltage V in. So that is approximately equal to V out upon V s. Now let us go for the second type of basic amplifier. So that is a current amplifier. In current amplifier the input signal is a current signal and again the amplified output signal is a current signal. So for a current amplifier we can say that it amplifier works like just like a current controlled current source current controlled current source. So in which the input impedance the input impedance of current amplifier should be low and output impedance of current amplifier should be high. So the ideal current amplifier amplifies input current signal amplifies input current signal and provides output amplified current signal. So that is proportional to input current signal and the proportionally constant that is a forward transfer ratio designated as AI. It is independent of input signal source impedance and output load resistance impedance. So and it is a ratio of output output current to input current. So at the input input side of this amplifier the current source IS is connected for this it is a internal resistance is connected in a parallel shown in parallel and the output circuit of this current amplifier it is shown by current source whose output resistance the output resistance of this amplifier. So that is shown in parallel with this current source output current of this amplifier I out. So that is equal to the load current IL. So IL flows through load resistance RL. So the current gain of this amplifier it is a ratio of amplified output current to the input current. So from this figure it is seen that if input resistance of this current amplifier is much less than the source input signal source resistance RS. So in that case effective input current of this amplifier is approximately equal to the input signal source current IS. If load resistance connected at the output of this current amplifier is much less than the output resistance of this amplifier RL. So in that case the load current IL the current flowing through load IL so that is equal to output current of this amplifier. So IL equal to AI multiplied by IN the input current IN. So where AI stands for forward current forward current the transfer ratio that is the current gain of this amplifier ratio of output current to input current. So current gain is AI equal to I out upon IN. So that is equal to load current upon input signal source current. So for ideal current amplifier the input impedance of this amplifier is ideally 0 and output impedance of this current amplifier is ideally infinity. Now third type of basic amplifier. So that is a trans conductance amplifier. In this amplifier the input signal is a voltage signal and the output signal of this trans conductance amplifier is a current signal. So trans conductance amplifier amplifies a low level input voltage signal and provides amplified output a current signal. So amplified output current signal is proportional to the input voltage signal the proportionality constant. So that is designated as GM. So that is called as a trans conductance is independent of input signal source impedance and output load resistance RL or output impedance of output load load impedance. So for a trans conductance amplifier ideally the input resistance RIN should be infinite as well as the output resistance of this trans conductance amplifier should be infinite. From this equivalent circuit of trans conductance amplifier trans conductance amplifier it is seen that if input source signal input signal source resistance RL is much less than input resistance of amplifier RIN then input voltage of this amplifier is approximately equal to input signal input signal source voltage VS and if load resistance RL is much less than output resistance of amplifier then output load current is equal to output current of this amplifier. So that is IL equal to I out that is equal to GM multiplied by VIN. So where GM stands for a trans conductance of this amplifier ratio of output to input output current upon input voltage. So that is GM multiplied by VS. So trans conductance GM equal to the load current IL divided by the input voltage input signal source voltage VS. Now third type of this amplifier that is trans resistance amplifier in this input signal is a current signal and output signal is a voltage signal. So trans resistance amplifier amplifies the input current signal and provides output voltage signal. So that is proportional to the input current signal proportionally constant. So that is designated as RM that is known as a trans resistance of amplifier ratio of output voltage to input current. So that trans conduct trans resistance RM is independent of input signal source impedance and output signal output load resistance RL. So from this equivalent circuit it is seen that if the input resistance RIN is much less than RS then I approximately equal to I S and if R out is much less than RL then output voltage of this amplifier that is equal to RM multiplied by I IN. So RM into I S. So gain or a trans resistance of this amplifier so that is designated as RM so that is ratio of output voltage to input current. So that is independent of input signal source impedance and output load impedance. So for a trans for ideal trans resistance amplifier so both input impedance and output impedance of amplifier are ideally 0. Now student can pause video here and think over this question and try to answer this question. State type of amplifier for transistor common emitter amplifier. For transistor common emitter amplifier the type of basic amplifier is a trans resistance amplifier because in trans resistance amplifier the input current is amplified at the output. So the input is current signal and output is a voltage signal. So as far as transistor amplifier is concerned transistor is a current control device. So it acts as a trans resistance amplifier in common emitter circuit configuration. Now this table shows the characteristic of ideal amplifiers basic amplifiers voltage amplifier current amplifier trans conductance amplifier and trans resistance amplifier their input signals and output signal and input impedance and output impedance and the ratio of output to input that is a trans per characteristics or trans per ratio or gain of this amplifier. So this is a reference. Thank you.