Electronic Engineering - Amplifiers?

Question

I would like to know - how to calculate the voltage output of a system that uses 4 amplifiers with gains of 100, 30, -15 and 8 dB( respectivlely),

The input voltage is 10 micro volts, what is the voltage output at each stage (Va, Vb, etc after each amp)

I would like to know the formulae for doing so and an explination of the units used within the calculations.


Thanks

Answer 1

Thanks to dmb06851. I made a correction from "to double the power you need to quadruple the voltage" to "to double the power gain you need to quadruple the voltage gain."

Let's make two typical assumptions: (1) that the gains stated are power gains, and (2) that the impedance of the input port is the same as that of the output port. Then you use these two formulas to calculate the gain at each stage:

VoltageGainDB = 2 * PowerGainInDB. That is to double the power gain you need to quadruple the voltage gain.

VoltageOut = VoltageIn * 10 ^ (VoltageGainDB / 20). That says the voltage gain in dB is defined as 20 * LOG10(Vout/Vin).

The warnings on oscillations are real.

Another specialty tech note on the attenuation (negative gain) stage that some textbook overlook. Gain calculation of this attenuation stage is a function of the waveform, especially one type of waveform called white noise (broad band random fluctuation) does not attenuate as does, say, a continuous sine wave (CW) signal. This makes the noise floor of the chain to run through a one-way-up non-negative gain only for both active gain and the passive attenuation stage. A thermo-attenuator filter can take the noise floor down, but it is not practical in a commercial amplifier.

Answer 2

Well u missed it a little . Any gain above approx. 20 dB will oscillate very loudly. 10 dB will give u 10 times your input. 20 dB will give u 100 times the input. and beyond that u will have oscillation. 30 dB will give u 1000 times the input. Of core there other input problems ,if the signal gets high enough to go into the non linear range there are distortion . The fet I like the most as it's input resist going into the non linear . Are u planing on the amps in series??
Good luck

Answer 3

Looking at the circuit values i guess its designed to run from a 9v battery. The output resistance is about 50 - 20 ohms so not a great match to your phones, but OK. It will take about 15mA when working, and put out about 2V rms giving 100mW If you want a better (louder) circuit consider using the excellent and cheap LM386 ic amplifier as shown in the "dual power king" circuit in the link. Just build 2 for stereo.

Answer 4

sciquest said "to double the power you need to quadruple the voltage."

I'm sure he was distracted when he typed that.

Power is proportional to the square of voltage, so to double the power the voltage has to be increased by a factor of 1.414 (and quadrupling the voltage would result in a power increase of 16 times.)

You both might find this of interest http://www.davidbridgen.com/dbnotation.htm