66.5.2. Increasing Gain

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 its limitation of the voltage gain.

In this part, we begin by reviewing the common emitter (CE) amplifier and its basic operation. The CE amplifier is known for its ability to amplify signals, but it has limitations regarding its voltage gain. The main component in the amplifier is the transistor, which requires a DC biasing voltage along with an input signal. A resistor (R_C) connected to the collector of the transistor serves a dual purpose: it keeps the transistor in the active region and converts the collector current to a corresponding output voltage. While the CE amplifier achieves good gain, there is potential for further enhancement by replacing the passive resistor with an active load component.

Think of the CE amplifier as a water pump. It can push water through a pipe to a certain height based on the pressure (voltage gain). However, if you want to raise the water to a much higher height (enhanced gain), you might need a more powerful pump (an active load) instead of just increasing the size of the pipe (passive load).

To come to the basic at the base, what we are doing is we are changing the voltage at the base or either you say base voltage or base to emitter voltage. And if you observe the based the current flowing through the base terminal say I instantaneous current having both DC as well as the small signal part as function of V, which is also having a DC part as well as a small signal part.

At the core of the CE amplifier, the base voltage (V_be) alters the base current (I_b). This current has both a direct current (DC) component and an alternating current (AC) component. As the voltage changes, it influences the collector current (I_c), which ultimately leads to a variation in the output voltage. Analysis of this relationship highlights how the transistor operates in response to voltage changes at the base, which is crucial in understanding how signals get amplified.

Consider a faucet connected to a water tank. Adjusting the faucet's handle (changing the base voltage) allows more or less water to flow out (changing the collector current), affecting the water level (output voltage) in the tank. Thus, minor adjustments can significantly affect the overall flow.

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 its actual the exponential relationship.

To understand gain in the context of a CE amplifier, it is vital to recognize how the DC operating point (quiescent point) serves as a reference. When we apply an AC signal, the device characteristics shift around this DC point, leading to changes in collector current and output voltage. The gain is essentially the ratio of output voltage change to input voltage change, visually interpreted through the slope of the I-V curve for the given transistor. Thus, the steeper the slope (or the change) in the output concerning the input signal, the higher the gain.

Imagine driving a car. If you press the accelerator (input voltage), the car speeds up (output voltage). The car’s acceleration rate (gain) varies at different speeds, being greatest when the car is moving steadily and can respond quicker to throttle changes.

If you want to further enhance the gain and that may be done by replacing this passive element by its active equivalent circuit. So, we can say that the maximum limit of this gain is the drop across this R resistance divided by thermal equivalent voltage.

This section discusses approaches to increase gain by substituting passive components with active ones, focusing on limits imposed by the voltage drop across the load resistor. The theoretical maximum gain arises from dividing this voltage drop (which cannot exceed supply voltage) by the thermal equivalent voltage. This highlights that while there are methods to enhance gain, practical constraints always exist due to power supply limits and efficiency.

This scenario is akin to a speaker system. You can only turn up the volume so much before the power supply (battery or outlet) cannot provide enough power to the speaker without distorting the sound (max gain). Just as speakers have a maximum volume they can produce, amplifiers have limitations based on their components.

What if we keep this point here and then we can try to decrease this and decrease this slope of the second mirror.

This part examines strategies for reducing the slope of the load line characteristic without raising the supply voltage, thus aiming to increase gain while keeping power dissipation in check. By adjusting transistor characteristics, it may be possible to maintain the quiescent point while enhancing slopes associated with gain, further demonstrating the potential benefits of utilizing active loads over passive loads.

Think of a seesaw. If one end is weighed down (maintaining the quiescent point), you can change the slope of the other end (angle of the seesaw) to obtain a better balance without altering the people sitting on either end. This is akin to adjusting gain without changing the overall circuit conditions.

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