P-Channel FET Amp: Deriving Output Voltage

  • Thread starter waht
  • Start date
  • Tags
    Amp Fet
In summary, the conversation discusses a schematic of a buffer amplifier that uses a p-channel fet as an input. The amplifier is part of a sweep oscillator that generates a ramp voltage using constant current. The p-channel fet amplifies the voltage from -5 to 5 volts in conjunction with another fet. The goal is to determine the output voltage as a function of the input voltage. The person who brought up the topic eventually solves the problem.
  • #1
waht
1,501
4
I came across a schematic of a buffer amplifier utilizing a p-channel fet as input. It is from a sweep oscillator generating a ramp voltage by charging C3 and C4 with a constant current, (current source is not shown, Q2 just acts as a switch or buffer). Now as the voltage rises linearly across C3 or C4 the gate of the p-channel fet Q3 picks it up and the amplifies it from -5 to 5 volts in conjunction with Q1.

So what I'm trying to accomplish is to derive an expression of the output voltage (-5 to 5) as a function of the input voltage at the gate Q3. P channel fets always confused me.
 
Last edited:
Engineering news on Phys.org
  • #2
Never mind I solved this, a good exercise though.
 
  • #3


The output voltage of this p-channel FET amplifier can be derived by considering the behavior of the p-channel FET in this circuit. The p-channel FET acts as an inverting amplifier, where the output voltage is the negative of the input voltage multiplied by the gain of the amplifier. In this case, the gain is determined by the ratio of the drain-source resistance of Q3 to the gate-source resistance of Q3.

Since the gate of Q3 is connected to the voltage across C3 or C4, the input voltage at the gate can be expressed as Vg = Q/C, where Q is the charge on the capacitor and C is the capacitance. As the voltage across C3 or C4 rises linearly, the charge on the capacitor also increases linearly. This means that the input voltage at the gate, Vg, is directly proportional to the ramp voltage generated by the sweep oscillator.

Now, considering the inverting nature of the p-channel FET amplifier, the output voltage can be expressed as Vo = -Vg * (Rd/Rg), where Rd is the drain-source resistance of Q3 and Rg is the gate-source resistance of Q3. Since the output voltage is amplified from -5 to 5 volts, we can set the ratio of Rd/Rg equal to 10. This gives us the final expression for the output voltage as Vo = -10Vg.

In conclusion, the output voltage of this p-channel FET amplifier is directly proportional to the input voltage at the gate and can be expressed as Vo = -10Vg. This circuit provides a simple and effective way to amplify a ramp voltage generated by a sweep oscillator and can be useful in various applications.
 

1. What is a P-Channel FET Amp?

A P-Channel FET (Field-Effect Transistor) Amp is an electronic circuit that uses a P-Channel FET as the main amplifying component. This type of amplifier is commonly used in audio and radio frequency applications.

2. How does a P-Channel FET Amp work?

A P-Channel FET Amp works by using the voltage applied to the gate of the FET to control the flow of current between the source and drain. As the gate voltage increases, the current also increases, resulting in amplification of the input signal.

3. What is the output voltage of a P-Channel FET Amp?

The output voltage of a P-Channel FET Amp can be derived using the following formula: Vout = Vin * (R2 / (R1+R2)), where Vin is the input voltage and R1 and R2 are the resistors connected to the gate and source of the FET, respectively.

4. How do you calculate the gain of a P-Channel FET Amp?

The gain of a P-Channel FET Amp can be calculated by dividing the output voltage by the input voltage, or by using the formula: Gain = R2 / R1, where R1 and R2 are the resistors connected to the gate and source of the FET, respectively.

5. What are the advantages of using a P-Channel FET Amp?

Some advantages of using a P-Channel FET Amp include low noise, high input impedance, and high gain. It also has the ability to operate at high frequencies and can be easily integrated into electronic circuits.

Similar threads

Replies
80
Views
3K
  • Electrical Engineering
Replies
14
Views
715
  • Electrical Engineering
Replies
12
Views
2K
  • Electrical Engineering
Replies
10
Views
1K
  • Electrical Engineering
Replies
26
Views
2K
Replies
9
Views
4K
Replies
17
Views
10K
  • Engineering and Comp Sci Homework Help
Replies
4
Views
2K
  • Electrical Engineering
Replies
3
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
34
Views
2K
Back
Top