47.4. Next Circuit with Practical Source Resistance

In the next slide, it is basically the same circuit except, here we are considering the source resistance R different from different from 100 k. So, if you see this circuit if you analyze this circuit with the bias current of say 0.5 mA and then if you try to analyze what will be the operating point.

In this chunk, we are looking at the introduction of a practical source resistance into the amplifier circuit. The practical source resistance is a crucial consideration because it affects the voltage drop across components and the overall behavior of the circuit. By adjusting the source resistance, we can analyze how the amplifier functions under realistic conditions as opposed to ideal ones, which helps us understand the importance of this parameter in circuit design.

Imagine you're trying to fill a glass with water. If the faucet has a high pressure, the water flows quickly. But if there's a blockage (like a practical source resistance), the flow rate reduces, and it takes longer to fill the glass. This is similar to how practical source resistance can limit the current flow in an amplifier circuit.

So we do have at the base we do have the 6 V of this V_BB, and then we do have the R_S and then base to emitter we do have a voltage drop. So, so I should say I am just circling it to indicate that this is representing V_BE(on).

In this segment, we determine the operating point of the amplifier considering the source resistance. The base voltage is initially set at 6 V. When current flows through the source resistance (R_S), there is a voltage drop due to the current flowing through it, which affects the base voltage. This means we need to recalculate the base-emitter voltage considering the drop, as it changes the conditions under which the transistor operates.

Think of a car trying to go uphill (transistor working). If the car has more weight (source resistance), it struggles more, and it slows down (drops voltage). This situation forces the car to adjust its acceleration (operating point) according to the incline (circuit bias conditions).

Note that even though the voltage here it is different from significantly different from 6 V bias here due to finite value of this source resistance. But since we do have the bias current here it is ideal quote and non-quote ideal of the same value of 0.5 mA the collector current and base current hardly gets affected.

Despite the changes in base voltage due to source resistance, the bias current remains constant at 0.5 mA, meaning the collector and base currents also remain unaffected. This indicates that the small signal parameters like the transconductance and output resistance stay the same, which in turn shows that the overall performance of the amplifier remains predominantly unchanged even with practical source resistance.

Imagine you're baking cookies and you switch ovens but keep the temperature settings the same. Although the ovens may behave slightly differently, if you maintain the right temperature (bias current), your cookie recipes (small signal parameters) will still turn out fine.

So now, you may recall that at this point if we give a give a stimulus of say v_x in the small signal equivalent circuit; then if we try to calculate what is the corresponding i_x.

In this chunk, the discussion focuses on calculating the output impedance while considering source resistance. We have to analyze the circuit by providing a small signal voltage (v_x) and evaluating how it translates to the corresponding output current (i_x). The output impedance is influenced by the source resistance and the other impedance parameters of the circuit. This helps us understand how the output of the circuit will behave under variations in load and input signals.

This can be likened to how the responsiveness changes in a water pressure system when you add a tap (representing the source resistance). If the tap is partially closed, it affects how much water (output) can flow out, and this is what we’re analyzing in terms of the circuit’s output behavior.

So the upper cutoff frequency of course, it will be defined by whichever is minimum.

In this portion, the concept of upper cutoff frequency is explained. The upper cutoff frequency defines the maximum frequency at which the amplifier can respond. It's determined by the interactions of various impedances in the circuit, including the source resistance and capacitance. The minimum of these cutoff frequencies sets the bandwidth of the amplifier, thereby defining the range of frequencies it can effectively amplify.

Consider a music equalizer that can adjust sound frequencies to desirable levels. If one frequency band is cut off more than others, it limits the range of sound you can enjoy, similar to how the upper cutoff frequency limits the amplifier's response.

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