31.1.2. Designing for Lower Gains

So far we have discussed about the design guidelines where our main objective there is to maximize the gain, voltage gain right. And, also the output swing we like to maximize and the power dissipation probably it is given value. And, this maximization of output swing of course, it is decided by the V_CC.

In the design of a Common Emitter (CE) amplifier, one of the main goals is to maximize both the voltage gain and the output swing. Voltage gain refers to how much an amplifier can increase the strength of a signal, while output swing is the range of output voltages that the amplifier can produce without distortion. Both of these factors are influenced by the supply voltage, denoted as V_CC. It’s essential to ensure these parameters are at optimum levels to ensure the amplifier functions correctly.

Think of an amplifier like a speaker at a concert. The voltage gain is like how loudly the speaker can project sound compared to the input signal, while the output swing is like how dynamic the music can be without distortion, allowing soft and loud parts to be heard clearly. Just like a concert requires careful management of speaker volume and clarity, designing an amplifier requires balancing gain and output capabilities.

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

When designing an amplifier that requires a lower gain than what is typically achievable, it is a crucial task to establish how to achieve the desired specifications without compromising the amplifier's performance. A lower gain can be desired for specific applications where excessive amplification could lead to distortion or other undesirable effects. The key is to figure out how to still maintain a good range of output swing while achieving the required gain.

Imagine you are adjusting the brightness of a lamp. If you set it too bright, it might wash out the details in the room, similar to how a high gain can distort the signal. Conversely, a dim lamp gives you better control over the ambiance. Similarly, in amplifier design, you want just the right gain that gives you clarity without overwhelming the signal.

So, the remedy for that instead of completely ignoring the CE or instead of completely bypassing this RE, we can partially bypass this resistor.

To achieve a lower gain without losing performance in bias point stability, one common approach is to partially bypass the emitter resistor (RE). This means that while part of the resistor is utilized to maintain stability, another part is bypassed to allow signals to pass more freely, which helps to maintain the desired voltage gain according to the design specifications. The careful selection of resistor values helps achieve a balance between gain and bias stability.

Consider a water pipe where you want to regulate water flow. If you close off too much of the pipe to restrict flow, you may cause pressure to build up, which can lead to a burst. However, by strategically placing a valve that only partially restricts the flow, you can control the pressure without fully stopping water flow. Similarly, partial bypassing of the resistor allows for optimal amplifier performance without losing control.

If we are looking for a circuit having this gain which is higher than the limit of the maximum gain we are achieving from a single stage for a given value of V_CC.

When a higher gain is required than what can be achieved through a single amplifier stage, engineers often use cascading. This involves connecting multiple amplifiers in series, where the output of one stage becomes the input for the next. Each stage contributes its gain to the overall circuit gain. However, proper design ensures that the input and output resistances are well-matched to avoid signal loss between the stages, ensuring maximum efficiency and performance.

Cascading amplifiers can be compared to a relay race in sports. Each runner represents an amplifier stage; while one runner can only cover a certain distance (gain), collectively they can cover much more (total gain). Just like the baton (signal) needs to be passed smoothly between runners for a successful relay, a properly designed cascading amplifier ensures signals transfer efficiently from one stage to the next.

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