66.6.1. Voltage Gain Limitations

This is a recapitulation or recalling whatever we know about CE amplifier and not only we will be talking about CE amplifier. But basic operation of the CE amplifier just to see that, what is it is limitation of the voltage gain.

In this section, we are reminded of the core principles of the common emitter (CE) amplifier. The CE amplifier uses a transistor where the input signal is fed at the base (input port) along with a DC component, ensuring the transistor operates in its active region. The resistor connected to the collector plays two crucial roles: it ensures the transistor remains in the active operation region and it converts the collector current into an output voltage.

Imagine a water tank with a tap at the bottom (the resistor). The height of the water (voltage) reflects how much you can turn the tap (the signal). If you turn it slightly, the water flows out easily, but if the height is capped (limit in voltage gain), you can’t get more water out even if you open the tap all the way.

Now, today what we are planning to cover it is under this active load amplifiers, we do have to start with we do have motivation of using this active load. Then, from that we will we will be talking about basic operation of amplifier having active load and their corresponding circuit analysis including, small signal model and then finding the gain or maybe intuitively explaining the gain and so and so.

We see how the base current (I_b) is influenced by the base-emitter voltage (V_be). The base current comprises both DC and small-signal parts which flows through the collector creating a voltage drop across the resistor. The outcome is an output voltage dependent on the input signal and the interaction between the variations of base current and collector current.

Think of this process as adjusting a dimmer switch for brightness. The base-emitter voltage is like turning the dial; small adjustments in the input signal can lead to significant changes in light output due to the characteristics of the bulb and electrical wiring (analogous to the transistor and resistor).

So, this resistor typically it is referred as load and it is if it is passive component which is providing linear I-V characteristics. Of course it works fine, but to some extent it is having limitation to give the voltage gain. I should say rather it is having good gain the CE amplifier basic CE amplifier it is having good gain. But in case if you want to enhance the gain further, then there is a scope of improving the gain and that may be done by replacing this passive element by it is active equivalent circuit.

While passive components like resistors provide a stable gain, there's a fundamental limitation on how much gain a basic CE amplifier can provide. If we want to exceed this voltage gain limit, we can replace the passive load with an active component—this switch can lead to better performance and higher gain values.

Imagine a bicycle (passive load) that can only go so fast. If you want to go faster, you might switch to a motorcycle (active load), which can achieve higher speeds without being limited in the same way.

Whenever we are giving a signal with respect to a DC operating point. So that means, we are changing the device characteristic up and down with respect to it is actual the exponential relationship, that makes the device characteristic namely the pull-down element characteristic it goes down or up.

When a signal is applied to a CE amplifier, it modifies the collector current around its quiescent point based on the input signal variations. The gain can be related mathematically to the slope of the transfer characteristics—a steeper slope correlates to a higher voltage gain realized in conjunction with the supply voltage and other circuit parameters.

Just like adjusting the volume on your stereo system affects how loud the music is, altering the DC operating point in an amplifier affects how the amplifier reacts to input signals, adjusting the 'gain' in the process.

If you consider the modification we are going to discuss it is more significant for common source amplifier if not important for common emitted amplifier.

The revisions we plan to implement are particularly beneficial for the common source amplifier. The need for replacing passive loads with active loads stems from achieving better voltage gain performance and the mitigation of the inherent limitations faced by passive components in amplifiers.

If you think of traditional cooking on a stove (passive load) versus using a microwave (active load), the microwave allows for quicker and more efficient cooking (higher gain) than the stove could provide within the same constraints.

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