# I have to design an audio preamplifier as a project, need help with OrCad simulation

#### Crisu Cris

Joined Oct 10, 2019
3
I have received an example schematic of how an audio preamplifier should look like. Based on that and a set of data(input voltage(7V), input resistance(60 ohms), output resistance(8 ohms) and amplification(2.1) ) I have to design my own schematic(which shouldn't be a lot different at the end of the day), but I don't quite understand how to do it.

Note: this is the 1st time I am designing a circuit, so I really don't know much about it.

My questions are:

*First, why is an AC power supply used as an input for the circuit?
*Which exactly are the values of the input and output voltage in this image(so I can calculate the amplification); The reason I am asking this is because I see those "0V" values
*Is there any simple way to explain how changing the circuit(adding resistors, changing a resistor to a very high value etc.) influences the circuit?
I am assuming the 81.54mV is the ouput voltage(still need an answer). All my attempts to modify the circuit (using my parameters and other transistor(we have to use a different one from BC109C) ) led to attenuation instead of amplification.

#### Jony130

Joined Feb 17, 2009
5,457
First, why is an AC power supply used as an input for the circuit?
Because someone wants to do the AC sweep analysis (frequency response/domain ).
The reason I am asking this is because I see those "0V" values
What you see is a DC voltage at a given point in a circuit, this is what you get when finished DC operating point analysis.
For example a DC voltage at Q1 collector is 1.425V. And you have 0 volts at the output because the capacitor blocks any DC voltage.
So, this is not the AC output voltage from the amplifier. You need to turn-on AC-sweep analysis to be able to see the amplifier AC gain.
Is there any simple way to explain how changing the circuit(adding resistors, changing a resistor to a very high value etc.) influences the circuit?
No.
I am assuming the 81.54mV is the ouput voltage
No, this is a DC voltage level at Q1 emitter (arross R3).

#### Crisu Cris

Joined Oct 10, 2019
3
Because someone wants to do the AC sweep analysis (frequency response/domain ).

What you see is a DC voltage at a given point in a circuit, this is what you get when finished DC operating point analysis.
For example a DC voltage at Q1 collector is 1.425V. And you have 0 volts at the output because the capacitor blocks any DC voltage.
So, this is not the AC output voltage from the amplifier. You need to turn-on AC-sweep analysis to be able to see the amplifier AC gain.

No.

No, this is a DC voltage level at Q1 emitter (arross R3).
So then whre should I read the Vin and Vout?

#### Jony130

Joined Feb 17, 2009
5,457
So then whre should I read the Vin and Vout?
Well, you Vin for an AC is 1V. And you need to anable the AC sweep analysis. To be able to see the results.

#### Crisu Cris

Joined Oct 10, 2019
3
Well, you Vin for an AC is 1V. And you need to anable the AC sweep analysis. To be able to see the results.
I had the AC sweep analysis enabled all this time.

#### crutschow

Joined Mar 14, 2008
32,934
I had the AC sweep analysis enabled all this time.
Then you need to plot the results for the output node.
I don't offhand know how that's done in OrCad.
Suggest you look in the help file.

#### JoeJester

Joined Apr 26, 2005
4,390
*First, why is an AC power supply used as an input for the circuit?

Audio is alternating current. Typically audio is 20-20000 kKz Here are two examples