41.1. Analog Electronic Circuits

Yeah. So, welcome after the break. So, we are talking about the, in fact, what we got it is the generalized model of CE and CS amplifier here. What it is having here it is the input signal source, having the source resistance of R_s, and then signal coupling capacitor C_1, and then if I consider this is the main amplifier where we do have the input resistance represented by this R_1. And then we do have voltage dependent voltage source, which means that this is the core of the amplifier, then we do have the output resistance R_2. And then C_3 and C_4, they are representing you know either C_π, C_gs or C_gd based on whether the circuit it is CE amplifier or CS amplifier.

In this chunk, we are introduced to the generalized model for Common Emitter (CE) and Common Source (CS) amplifiers. The input comprises a signal source and its resistance (R_s), followed by a coupling capacitor (C_1). The amplification process begins at the input resistance (R_1) of the amplifier which also includes a core voltage-dependent source functioning as the active component of the amplifier. In terms of design, the output resistance of the amplifier is noted as R_2. Capacitors C_3 and C_4 play a crucial role in representing the effects of various capacitances that are inherent in the transistor inputs (C_π, C_gs, C_gd depending on whether we're discussing CE or CS configurations). This model helps in analyzing the amplifier's behavior at high frequencies.

Consider the amplifier as a musical concert where the input signal is akin to an audience's cheers. The source resistance (R_s) can be thought of as the noise from the crowd that might drown out the cheers. The coupling capacitor (C_1) acts like a sound engineer who filters the unnecessary noise so that only the cheers (signal) are processed by the main stage (main amplifier), ensuring that they reach the output (R_2) as clearly as possible.

So, this particular capacitor it can be converted into two equivalent capacitance; one is for the input port, the other one is for the output port. And then, the input port part coming out of the C_4 it is what we said is that C_4(1 ‒ A) or in this case A is equal to V_out/V_in. If you see here we are putting a ‒ sign here assuming that the polarity of the voltage dependent voltage source, here it is +ve. On the other hand, so this is the contribution coming to the input port earlier we used to call C_1. Now, let me put a different name C_4-in coming due to C_4.

This chunk discusses how capacitors in the circuit can be treated as equivalent capacitances. Specifically, one capacitor provides support for the input port and another for the output port. The relation C_4(1 - A) helps to define how much of the output voltage (V_out) affects the input capacitance (C_4-in). A negative sign indicates a phase reversal in the output signal, which is characteristic of CE and CS amplifiers since the output is often inverted. You should also note that C_4 used to be referred to as C_1 at times but for clarity in different contexts, it is renamed.

Think about an orchestra tuning before a performance. The input capacitance is like the sound adjustments made at the microphones (input port) and the output capacitance is like the adjustments made to the speakers (output port). Together, they ensure that the sound produced (output voltage) resonates well with the venue, while the difference in phase (inversion) is akin to the delay that may occur in sound reaching different areas of the hall.

So, likewise the output port capacitance coming due to C_4, let you call this is C_4-out. So, this is equal to C_4(1 + A). This capacitance it is coming in addition with C_3 as a result we are getting the net input capacitance C_in = C_3 + C_4(1 - A). On the other hand the output capacitance net output capacitance of course, we do have C_L. So, the C_L is coming as is plus this part namely C_4-out.

In this section, the output capacitance (C_4-out) relates back to the input via its gain (A). Specifically, we see that C_4-out can be represented as C_4(1 + A), showing the contribution of the amplifier gain on the output side. The net input capacitance (C_in) is calculated by adding the contributions from C_3 and the modified output capacitance from C_4. This cumulative evaluation helps set up the analysis framework to understand how the whole circuit behaves in the frequency domain, where high frequency effects come into play.

Imagine adjusting the volume (output capacitance) of a speaker based on the sounds it receives (input capacitance). The gain (A) modifies how much influence the sound has when it’s played back through the speakers. If the audience is particularly energetic (high gain), it amplifies even the smallest sounds, resulting in a higher volume at the output.

So, in summary what we have is at this node we do have the C_in and then at this node we do have the net C_out. Now, to get the frequency response of this circuit namely starting from this point till the primary output what we have it is we do have one network here and then we do have of course, the main amplifier starting from this point to this point and then at this point we do have the C_out.

The final part of the analyzed section discusses the overall frequency response of the circuit beginning from the input to the output. It summarizes the important elements consisting of the combined input capacitance (C_in) and output capacitance (C_out). This effectively is the base for extracting the frequency response of the whole circuit, which is a fundamental aspect in amplifier design as it helps in understanding how the amplifier will perform with varying input frequencies.

Think of a video game console out front (input) and the television set in the back (output). The network between them includes all the processing (main amplifier). The video game stream needs to be clear without any lag (frequency response). Here, you want to ensure that the signal is strong enough at both ends for an immersive experience. Any disturbances or delays can affect gameplay, similar to how frequency responses can impact audio signals.

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