# What Is The Maximum Useful AC Output Voltage?

#### elec_eng_55

Joined May 13, 2018
214
Hi:

I have computed the maximum ac output voltage as follows:

Lesser of: Ic * (rc + re) = 1mA * (2.91K + 0) = 2.91 Vpk

Or : Vce = 5 Vpk

Therefore Vmax = 2.91 Vpk

But this output is not realistic as it is badly distorted. In this
example, Vce = 5 volts, but it can't be utilized since the
maximum useful output seems to be approx. 215 mVpk
per the sim.

Why do textbooks and lecturers make these calculations?

What is the maximum useful ac output voltage?

Thanks,

David

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Last edited:

Joined Mar 10, 2018
3,506
Here is a sim done for another poster, basically seems to work pretty
good. Curves are input and at the load. After I posted this tried it with
a 3904, essentially same results.

Attached.

Regards, Dana.

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#### Alec_t

Joined Sep 17, 2013
10,267
Remove the emitter capacitor and you can get about 6.5V pk-pk out.

#### Bordodynov

Joined May 20, 2015
2,354
When using a cascade circuit with a common emitter and without negative feedback, you can get the limiting amplitude of the signal, but the output signal will be greatly distorted.Already at the input signal 10mV distortion will be 10%.You can get near-limit voltages by entering negative feedback.But the voltage gain will be less and sometimes much less.Discard the emitter capacitor from the circuit.The gain will be about 2.8 times.Input a signal with an amplitude of 1.1V and you will get an amplitude almost as in theory.
See

#### elec_eng_55

Joined May 13, 2018
214
I still do not have an answer to my two questions pertaining to the original
schematic:

Why do textbooks and lecturers make these calculations?

What is the maximum useful ac output voltage?

#### Bordodynov

Joined May 20, 2015
2,354
I believe that the calculations given by your teachers make a lot of sense. Beginners in circuit design often make mistakes. They take a kilohm resistor in the collector and load this cascade with a low-impedance load (4–8 ohms), and then wonder why there is no amplification (small output signal)! Some do not model the circuit and solder the real circuit. And they are very upset.

Joined Mar 10, 2018
3,506
You would proceed with calculations for two cases, transistor almost
cutoff, no current thru it, and other case close to saturation, most current
thru it, and that will determine what the range of voltages will be seen at
output node.

Calculations are done to insure a circuit does what it was designed to do,
typically over T and V and component variation.

Regards, Dana.

#### Alec_t

Joined Sep 17, 2013
10,267
What is the maximum useful ac output voltage?
That depends on how much distortion is acceptable in a particular situation. For audio work, low distortion is necessary, but if you simply wanted to use the output as a switching signal then distortion would generally be irrelevant and you could get nearly the full supply voltage out of the circuit.