31.1. Common Emitter Amplifier (Contd.) Design Guidelines (Part B)

Welcome back after this short break and let me continue the Design Guidelines for CE Amplifier. 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 a given value. And, this maximization of output swing of course, it is decided by the V_CC. Likewise, the maximization of the gain is also decided by this V_CC and thermal equivalent voltage and of course, the output swing.

This chunk discusses the primary goals when designing a Common Emitter Amplifier (CE amplifier). The two major objectives are maximizing the voltage gain and output swing while managing power dissipation, which is often predetermined. The output swing is crucial for ensuring that the amplifier can handle the desired signal variations without distortion, and it directly depends on the supply voltage (V_CC) and thermal considerations. Understanding these elements is essential for successfully designing an amplifier that meets specific requirements.

Think of designing a speaker system. You want the sound (gain) to be loud and clear (output swing), but you also have to consider how much power (dissipation) the system can handle without overheating. Just like adjusting the volume and power limits of a speaker, adjusting V_CC helps control how much 'amplification' you can achieve in your amplifier design.

So, typically we like to keep the output swing towards its maximum as much as possible, but definitely there may be a requirement where say we require only a 20 gain, unnecessarily we will not be looking for say 230 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 explains that while high gains are often desirable, there are scenarios where a lower gain (like 20) is sufficient. The challenge arises when designing the circuit to achieve this specified lower gain without compromising the overall performance or stability. The design considerations will vary based on the desired gain, and specific design guidelines can be followed to ensure a stable operating point despite reduced amplification.

Imagine you are tuning a radio. Sometimes, you want a signal just good enough to listen to music (lower gain) without blasting it to maximum volume. This is similar to designing an amplifier for a specified gain—measuring and setting the right levels to achieve clarity without distortion or unnecessary amplification.

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

In this part, the discussion revolves around mitigating issues that arise when designing an amplifier circuit. Instead of bypassing the Emitter Resistor (R_E) completely, which could lead to gain instability, the design can include a partial bypass. This strategy helps maintain a stable operating point by ensuring that some resistance remains in the circuit, which stabilizes the voltage across the transistor and prevents large variations in gain due to changes in β (beta), a parameter that describes the transistor's amplification capability.

Imagine a water faucet. If you partially close the faucet (partial bypass), you can control the flow of water better, compared to fully opening or fully closing it which could lead to overflow or no flow at all. Similarly, partially bypassing R_E allows you to control the gain better and ensure smooth operation of the amplifier.

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... So, there are two possibilities probably we can replace this resistor by an active device... cascade to an amplifier to get total gain maybe multiplication of the 2 individual stages gain.

Cascading refers to connecting multiple amplifier stages in sequence to increase overall gain. When seeking a gain higher than what a single CE amplifier can provide, designers often resort to connecting two or more amplifier circuits together. This method allows the total gain of the system to be the product of each stage's gain, effectively multiplying their amplification capabilities. However, it is essential to calculate the resistances properly to ensure that the connection between the two stages does not negatively impact performance.

Think of a relay team in a race. Each runner (amplifier stage) has their own strengths, but when combined, they can cover more distance in less time (gain). If each runner does well, the entire team benefits, and that's the concept behind cascading amplifiers to achieve much higher overall gain.

Now, if I replace these 2 amplifiers by their corresponding model for signal, small signal model... So, the voltage coming here is the A × v_1; this v that voltage it is only here.

This section emphasizes the importance of understanding how the input and output resistances of each amplifier stage influence overall circuit behavior. In a cascaded configuration, the output resistance of the first amplifier can affect the input resistance of the second. One must carefully account for these resistances when calculating overall gain and ensuring signal integrity. Properly matching these resistances can maximize efficiency and reduce signal loss.

Imagine a chain where each link represents an amplifier stage. If one link (output resistance) is too heavy or large, it slows down the entire chain (overall gain). By making sure each link is appropriately sized (matching resistances), the chain moves smoothly and efficiently, much like ensuring signal flow between cascaded amplifiers.

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