31.1.5. Example Calculations

So, in case if we are looking for an amplifier having again, which is less than this limit, then how do you design? And, of course, having higher swing it is always better.

In designing amplifiers, we often seek specific gains. If the required gain is lower than the maximum limit (for instance, 230), we must adapt our design. This adaptation can involve selecting suitable resistor values or configurations that allow for a stable operating point, even at lower gains.

Imagine you have a car that can go up to 230 km/h, but you only need to drive at 20 km/h. You wouldn't just drive at a slow speed; you'd adjust the gear settings or throttle to ensure the car runs smoothly and efficiently at that lower speed.

For example, if you are looking for say this ratio to be say or say this gain, it is given to us is a 20. And, if you are taking this ratio to be 20...

When aiming for a specific gain, such as 20, the resistor values in the circuit must be chosen carefully. The voltage drop across these resistors will influence the gain, stability, and sensitivity to fluctuations in components like beta (β). If the resistors are not chosen correctly, the stability of the circuit could be compromised.

Think of baking a cake where the right proportions of flour, sugar, and eggs are crucial. If you don’t use the correct amounts, the cake may not rise adequately or could fall flat, just like improper resistor values can lead to a poorly functioning amplifier.

So, what may be the remedy for that instead of completely ignoring the C or instead of completely bypassing this R , what you can do we can partially bypass this resistor?

To maintain stability in the amplifier while achieving desired gain, one effective strategy is to partially bypass the emitter resistor (RE) with a capacitor (CE). By doing this, part of the signal can bypass this resistor at higher frequencies, improving gain without entirely losing the stability benefits that RE provides against variations in β.

Imagine a highway with a toll booth. While you could completely avoid the toll booth (representing the resistor) and take a longer backroad, partially using the toll booth allows for a faster trip while still benefiting from the main road’s structure and safety.

So, the next thing is that in case if we are looking for a circuit having this gain, which is higher than the limit of the maximum gain we are achieving from single stage for a given value of V.

When a single stage amplifier can't achieve the desired gain, one solution is to cascade multiple amplifier stages. By connecting them in series, the overall gain can be the product of the individual gains. Each stage amplifies the signal before passing it onto the next, effectively allowing for higher overall amplification.

Consider filling a bucket with water using several small cups. Each cup represents an amplifier stage. While one cup may not bring the water level to your desired point, using several cups in succession allows you to reach it quickly and efficiently.

So, let's see how do we find the gain of that circuit? Again we are going back to the circuit analysis...

To find the total gain of cascaded amplifier stages, one must account for the individual gains of each stage and the resistances that affect signal attenuation between stages. This requires careful calculations to ensure that the combined output reflects the gain desired, considering how output resistance from one stage can affect the input of the next.

It's like a chain of dominoes; for each domino (amplifier stage) to fall effectively, the right amount of push (gain) must be considered. If one domino is too far apart from the next (due to input and output resistances), the series might not work as intended.

When it is the same type or does it mean is that, we are analyzing these 2 circuits as voltage amplifier...

Amplifier stages in a cascade do not need to be of the same type. For instance, the first amplifier could be a transconductance amplifier, which converts current to voltage, while the second might be a transimpedance amplifier, which converts voltage to current. This versatility provides flexibility in designing complex circuits tailored for specific applications.

Imagine a relay race where each runner uses a different running style. The first runner might sprint (current), while the next jogs (voltage). Despite the differences, they still work together in harmony to finish the race effectively, just like these amplifiers in a cascade.

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