# Low and High Frequency Signal Equivalent

• DODGEVIPER13
In summary, the conversation involves drawing the low frequency and high frequency small signal equivalent circuits for an amplifier and deriving the corresponding 3dB frequency formulas. The maximum gain of the amplifier is also discussed, with a request for a magnitude plot. The circuit in question is a PMOS with heavily bypassed source, and the main capacitive contributors to its high-frequency response are Cgd and Cgs.
DODGEVIPER13

## Homework Statement

Assume λ≠0. Also bypass capacitor is very large compared to coupling capacitor.
a.) Draw the low frequency small signal equivalent and derive lower 3dB frequency formula.
b.) Assuming transistor high frequency capacitances, draw the high frequency small signal equivalent and derive upper 3dB frequency formula
c.) What is the maximum gain of this amplifier? Finally draw the magnitude. Oh and the circuit is in the thumbnails

I only need help with b and c

## The Attempt at a Solution

I have figured out part a and have drawn part b I have it drawn such that it goes from ground to Vs and then to Rs, with cc1 shorted, and then Rg in parallel with Vs then Cgs in parallel with Rg and Cgd in series with the ground terminal of the mosfet. I have the dependent source connected to ground and going up as the circuit is PMOS at GmVsg and the ro, Rd, and Rl are all in parallel and on Vo. Now what I am wondering is how do I get Ceq I have Req=ro in parallel with Rd in parallel with Rl. I mean there isn't a cap on the output side so I am confused on how to find the upper frequency I know the formula is 1/(2pi(τ)) where τ=RC I think Ceq=Cgs+Cgd. Sorry for the terrible writing quality but I don't have a way to scan in my circuit nor a way to draw it.

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To do part (b) you probably want to simplify the usual 6-or-so-capacitance model for the MOSFET. I suggest cutting down to one or two. (Some of the other capacitances are removed by virtue of the heavily bypassed source anyway).

The biggest contributor to limiting the high-frequency response is probably Cgd, aka the "Miller" effect. Any voltage variations in the drain get fed back to the gate thru this capacitance.

The second-biggest contributor is probably Cgs which, since the source is heavily bypassed, is just the capacitance from the gate to ground. So just parallel Rg with Cgs.

Thanks man I got it figured it out

## 1. What is the difference between low and high frequency signals?

Low frequency signals have a lower number of cycles per second, while high frequency signals have a higher number of cycles per second. This means that low frequency signals have a longer wavelength and high frequency signals have a shorter wavelength.

## 2. How do low and high frequency signals affect communication?

Low frequency signals are able to travel longer distances and penetrate obstacles, making them useful for long-range communication. High frequency signals, on the other hand, are better for short-range communication and can carry more information due to their shorter wavelength.

## 3. What types of devices use low and high frequency signals?

Low frequency signals are commonly used in devices such as radios, telephones, and power lines. High frequency signals are used in devices such as wireless routers, cell phones, and satellite communication systems.

## 4. Can low and high frequency signals be converted into each other?

Yes, it is possible to convert low frequency signals into high frequency signals and vice versa using electronic devices called frequency converters. However, there may be some loss of signal quality during the conversion process.

## 5. What are some examples of low and high frequency signals in everyday life?

Some examples of low frequency signals in everyday life include the hum of a refrigerator, the ticking of a clock, and the sound of a person's voice. High frequency signals can be found in things like remote controls, microwave ovens, and wireless headphones.

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