22.1.3. Examples of Voltage Amplifiers

Say for example, the first one it is; it consist of BJT, second one it is having a MOS transistor. And for each of the cases what we have here it is, the DC voltage source and then of course the ground and along with the biasing element resister. The transistor it is in this case the BJT is kept in the active region of operation with the help of V_BE, the DC voltage here. And then on top of the DC voltage we do have the small signal called v_be. And on the other hand here for the transistor having the MOSFET, it is again it is having a DC voltage source and then of course the ground, main ground and then we do have the biasing resistor call R_D connected to the drain of the transistor. And then at the gate, we are applying a DC voltage call capital V_GS on top of this DC voltage we are applying a small signal.

This chunk introduces two types of voltage amplifiers: one using a Bipolar Junction Transistor (BJT) and the other using a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). Both circuits include a DC voltage source and a small signal voltage superimposed on the DC voltage. The BJT is biased to remain in its active region with a base-emitter voltage (V_BE), while the MOSFET uses a gate-source voltage (V_GS) for bias.

Think of the BJT and MOSFET as two types of water taps. The BJT is like a tap that lets water flow based on how much you turn the handle (the V_BE), while a MOSFET is like a tap that opens with a remote control (the V_GS). Both taps can let water flow in response to different inputs (the small signals), but they operate in different ways.

So, the if you see the voltage here along with this v_gs; the voltage here we are applying is, it is having a DC part as well as the small signal part. So, we can say V_GS. So, likewise for the BJT circuit on the other hand we do have the V_BE. And it produces a voltage at the output, which is having a DC part plus small signal part called v_out. So, same thing out here also, it is having a DC part and the small signal part.

In both circuits (BJT and MOSFET), the input voltage is a combination of a DC level and a small AC signal. The output will also consist of a DC component plus an AC signal (v_out). This means that while working with these amplifiers, engineers often look more closely at the changes caused by these small signals while keeping the DC level stable.

Imagine a radio tuned to a station. The main tune (DC part) is constant, while the small fluctuations in sound (small signals) are what you hear as music. Just like the radio, amplifiers process the main signal and add variations to produce a desired output.

So, whenever we are going for say model or linearization basically what you do; as I said that the transistor we are keeping in the appropriate region of operation, and then after that we try to find what is the relationship between the applied voltage here to whatever the corresponding output we are getting. So, in our main operation the DC conditions are kept intact, only the small signal part changes with time and then the corresponding effect we observe here.

Linearization is a technique used to simplify the analysis of circuits. By keeping the transistor in its active region, the relationship between the input (applied voltage) and output (resulting voltage) can be better understood. This relationship primarily involves examining how changes in the small signal alter the output voltage, while the larger DC conditions remain untouched.

Consider a seesaw on a playground. If one end represents the DC voltage and the other end represents the small signal, the seesaw balances on a pivot (active region). Only small adjustments (the small signal) will move it up and down around this resting point, allowing us to study those small changes without altering the fundamental balance (DC part).

Whenever you are talking about modelling of voltage amplifier what does it mean is; we like to change the circuit, whole circuit by an equivalent circuit or you can say equivalent linear circuit, which primarily correlates the small signal output with respect to whatever the small signal input we are giving.

Creating a linearized model involves replacing the actual circuit components with an equivalent circuit that still represents the input-output relationship accurately. This simplifies the analysis, allowing for easier calculations and predictions about how the amplifier will perform under different conditions.

Think of this model as creating a simplified map for a complex city. Instead of navigating every street, you create a straightforward route that captures the major landmarks (the functional relationship between input and output) without getting bogged down in intricate details.

So, we may say that to again to summarize what we said is; that we like to get the input to output signal relationship primarily. So, this is the say, you may say that input signal and this is the corresponding output. So, the input signal of course, it may be coming from the signal source it need not be same. And since we are expressing this output in terms of the input signal, we the simplified model must capture one basic element called voltage dependent voltage source.

Understanding the key parameters of a voltage amplifier is crucial for predicting its performance. By summarizing how the input signal affects the output and representing that relationship with voltage-dependent sources, engineers can create models that effectively demonstrate how changes in the input will influence the output.

Imagine you are baking a cake, where the input ingredients (sugar, flour, eggs) will affect the final taste. The relationship between your ingredients and the finished cake is like the voltage amplifier's relationship between input and output signals.

Now, what are the other elements we require? Note that the basic purpose of having this model of the voltage amplifier is that to simplify the circuit; but at the same time it must represent whatever the necessary information we are looking for. So, apart from the gain next thing is that, the loading effect at the output port and the input port.

In voltage amplifiers, input and output resistances are essential for determining how the amplifier interacts with connected circuits. The loading effect occurs when the output voltage is influenced by the load connected, while the input voltage may change depending on the characteristics of the input source. Proper modeling must take these resistances into account to ensure accurate performance predictions.

Think of this as a water pipe system where input and output resistances represent the size of the pipes. If the output pipe is too small (high resistance), it will constrain the flow (output voltage), similar to how a small load affects voltage output in circuits. Conversely, a larger input pipe (low resistance) will allow more water in, enhancing overall flow (input voltage).

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