Voltage gain for amplifier (JFET) in common gate

In summary, the capacitor works as a short circuit regarding the altern current. Kirchhoff's law is used to get $v_{DS}$. v_{DS}=-v_i+v_o=v_{GS}+v_o. v_o is then found by solving for it.
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Homework Statement
Prove that the voltage gain for the following amplifier in common gate is $$A_V=\frac{(g_mr_d+1)R_L}{R_S(g_mr_d+1)+R_L+r_d}$$
Relevant Equations
* We use the Hybrid model to analyze the transistor.

$$A_V=\frac{(g_mr_d+1)R_L}{R_S(g_mr_d+1)+R_L+r_d}$$
Screenshot from 2021-12-18 17-19-09.png


First the hybrid model, I assume the capacitor works as a short circuit regarding the altern current:
Screenshot from 2021-12-18 17-24-05.png


$$A_V=\frac{v_o}{v_i}$$
$$v_i=-v_{GS}$$
$$v_o=-i_LR_L$$
$$i_L=g_mv_{GS}+\frac{v_{DS}}{r_d}$$

Now I use the Kirchhoff's law to get $v_{DS}$. I consider this close loop:
Screenshot from 2021-12-18 17-58-04.png


$$-v_i-v_{DS}+v_o=0$$
$$v_{DS}=-v_i+v_o=v_{GS}+v_o$$
$$i_L=g_mv_{GS}+\frac{v_{GS}+v_o}{r_d}$$
$$v_o=-i_LR_L=-\left(g_mv_{GS}+\frac{v_{GS}+v_o}{r_d}\right)R_L$$
Now I just try to get $v_o$ from that equation:
$$v_o=-g_mv_{GS}R_L-\frac{v_{GS}+v_o}{r_d}R_L$$
$$v_o=-g_mv_{GS}R_L-\frac{v_{GS}}{r_d}R_L-\frac{v_o}{r_d}R_L$$
$$v_o+\frac{v_o}{r_d}R_L=-g_mv_{GS}R_L-\frac{v_{GS}}{r_d}R_L$$
$$v_o\left(1+\frac{R_L}{r_d}\right)=-g_mv_{GS}R_L-\frac{v_{GS}}{r_d}R_L$$
$$v_o=\frac{-g_mv_{GS}R_L-\frac{v_{GS}}{r_d}R_L}{1+\frac{R_L}{r_d}}$$
$$v_o=\frac{-v_{GS}R_L\left(g_m+\frac{1}{r_d}\right)}{1+\frac{R_L}{r_d}}$$
$$v_o=\frac{v_iR_L\left(g_m+\frac{1}{r_d}\right)}{1+\frac{R_L}{r_d}}$$
$$A_V=\frac{v_o}{v_i}=\frac{R_L\left(g_m+\frac{1}{r_d}\right)}{1+\frac{R_L}{r_d}}\cdot\frac{r_d}{r_d}$$
$$\boxed{A_V=\frac{R_L\left(g_mr_d+1\right)}{r_d+R_L}}$$

The answer given by my teacher is:
$$A_V=\frac{(g_mr_d+1)R_L}{R_S(g_mr_d+1)+R_L+r_d}$$

As you can see I am missing a whole term in the denominator. I do not understand how ##R_S## gets into the equation.
 
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FAQ: Voltage gain for amplifier (JFET) in common gate

1. What is the purpose of an amplifier in a circuit?

An amplifier is used to increase the strength or amplitude of an electrical signal. This is important in electronic devices because many signals are too weak to be useful without amplification.

2. How is voltage gain defined for an amplifier?

Voltage gain is defined as the ratio of output voltage to input voltage for an amplifier. It is typically measured in decibels (dB) and represents the amplification factor of the signal.

3. How does a JFET (junction field-effect transistor) work in a common gate amplifier?

In a common gate amplifier, the JFET is connected between the input and output circuits. It works by controlling the flow of current between the source and drain terminals, which in turn affects the output voltage. The gate terminal is used to control the current flow, making it a useful component in amplification.

4. What factors affect the voltage gain of a common gate amplifier using a JFET?

The voltage gain of a common gate amplifier using a JFET can be affected by several factors, including the bias voltage, the input and output impedances, and the transconductance of the JFET. These factors can be adjusted or selected to achieve the desired voltage gain for the circuit.

5. Can the voltage gain of a common gate amplifier using a JFET be greater than 1?

Yes, the voltage gain of a common gate amplifier using a JFET can be greater than 1. In fact, this is the desired outcome of an amplifier – to increase the strength of the signal. The amount of voltage gain achieved will depend on the specific circuit design and components used.

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