31.1.3. Circuit Adjustments for Gain

So far we have discussed about the design guidelines where our main objective 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. Likewise, maximization of the gain is also decided by the V_CC and thermal equivalent voltage and of course, the output swing.

This chunk discusses the key goals in designing circuits, particularly amplifiers. The main objectives are to maximize the voltage gain and the output swing. The voltage gain is a measure of how much the amplifier increases the input signal, while output swing refers to the range of output voltages that the amplifier can produce. The value of the power supply voltage (V_CC) and the thermal properties of the circuit affect both the gain and output swing. Designers must find a balance to achieve the best performance.

Think of an amplifier like a speaker system at a concert. Just as you want the speakers to make the sound louder (gain) while still being able to deliver clear sound without distortion (output swing), in electronic circuits, you want to maximize the voltage gain while keeping the output voltage within a usable range.

So, typically we like to keep the output swing towards its maximum as much as possible, but definitely there may be a requirement where we require only 20 gain. So, in case if this gain is specified and if it is less than this limit, then what may be the design guidelines or design procedure to achieve that.

This chunk explores scenarios where the designer needs to achieve a specific gain, such as 20, which may be lower than the maximum gain the circuit can provide (230 in this context). Here, the focus is on how to adjust the circuit design to meet less demanding gain specifications while still ensuring that other performance parameters, like output swing, are maintained at acceptable levels.

Imagine a car designed to go 200 km/h, but someone only needs it to reliably cruise at 80 km/h. The designer must adjust the engine's parameters to ensure it performs efficiently at this lower speed without compromising safety and reliability.

Instead of completely ignoring the C_E part or completely bypassing this R_E, we can partially bypass this resistor. We can have 2 parts; one is the say R_E1 and R_E2 and only one of them is getting bypassed by C.

In this chunk, the discussion revolves around optimizing gain by controlling the bypassing of emitter resistors (R_E). By partially bypassing R_E with a capacitor (C), the designer can manage the ratio between the two resistors (R_E1 and R_E2) to maintain the desired gain while also ensuring stability against variations in transistor beta (β). This technique allows flexibility in achieving target performance metrics.

Consider making a smoothie. If you blend everything without adding liquid (bypassing the resistance) it might become too thick (high resistance), impacting the taste. However, if you add just the right amount of liquid (partially bypass) to achieve the perfect consistency, you maintain the good flavor (gain) while ensuring it isn’t too thick or runny.

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, we can probably cascade two amplifiers together to get total gain, which is the multiplication of the 2 individual stages’ gain.

Here, the concept of cascading amplifiers is introduced, where two amplifier stages are connected such that the output of the first feeds into the second. This method allows the overall gain to be the product of the individual gains of each stage, which can significantly increase the total gain beyond what a single stage can provide. This is a common design strategy in practical amplifier circuits to achieve high gains.

Think of cascading amplifiers like stacking building blocks. Each block can be thought of as a stage of amplification. Alone, a single block might only be a few inches tall (limited gain), but by stacking multiple blocks on top of each other, you can create a tower that reaches impressive heights (higher overall gain).

So, overall gain is the individual gain of the stages multiplied by this attenuation factor. The attenuation factor is coming from the output resistance of the first stage and the input resistance of the second stage.

This chunk explains how to calculate the overall gain when amplifiers are cascaded. It highlights that the overall gain is not just the product of individual stage gains; it is also affected by an attenuation factor, which arises from the interaction of the output resistance of the first stage and the input resistance of the second stage. Understanding this factor is crucial for accurately predicting amplifier performance.

Imagine a football team trying to score points. Just because each player (stage) is skilled (gain) doesn’t guarantee that the team will score many goals because they may have to pass (attenuation) which can reduce their chances. The overall effectiveness of the team is influenced by both the individual player's skills and how well they work together.

The procedure can be deployed for many stages, and the most important thing is that these 2 stages need not be of the same type.

This chunk discusses the flexibility in cascading different types of amplifiers (such as transconductance and transimpedance). It emphasizes that the analysis technique does not rely on both stages being the same type, which opens up various design possibilities. This versatility allows for tailored designs to meet specific application requirements.

Just like assembling a team for a project, having members with different skills (engineer, designer, marketer) can lead to a more successful outcome than having all team members with the same expertise. Each member contributes uniquely, enhancing the project's overall success.

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