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a question about local feedback

hacknet

Member
2002-11-26 4:28 pm
sg
i was pondering bout my next amp project and it hit me that i didn`t have an idea what local feedback did to the output impedance of the stage. and if implemented in an output stage, what kind change in output impedance would i see...? is it very much like global feedback?

thanks!
 
3 decibels (power) = x 2 = 6 decibel-volts.

dBV are double (duodecibels?) because doubling voltage doubles current into a resistance, so power actually goes up by a factor of four (i.e., +6dB).

At any rate, linear negative feedback reduces Zo, distortion, gain and, if connected properly, increase Zin all by the same factor. This factor is the amount of feedback.

Tim
 
3 decibels (power) = x 2 = 6 decibel-volts.

Not quite.... 3dB increase for voltage means that power also increase 3dB

dB for voltage = 20 * LOG(V2/V1), so if V2/V1 is 2, (double voltage) it gives 6dB

dB for Power = 10 * LOG(P2/P1), so if V2/V1 is 2, (double power) it gives 3dB

However 3dB for voltage and 3dB for power is the same thing

V2/V1 = 1.4142 gives 3dB and an increase of 1.4142 in voltage gives an increase of 2 times in power, (P ~ V^2) and 2 times in power is also 3dB.

Back to the original question, 3dB feedback is 1.4142 times in voltage so the decrease in output impedance will be 1.4142 times, (I assume that what is meant with amount feedback here is how much the gain decreases)

Regards Hans
 
By numbers alone, 3dB (power) = factor of two = 6dBV.

Tim I understand what you write but it is not correct, as I wrote:

"dB for voltage = 20 * LOG(V2/V1), so if V2/V1 is 2, (double voltage) it gives 6dB

dB for Power = 10 * LOG(P2/P1), so if V2/V1 is 2, (double power) it gives 3dB"

This is the definition of dB.

What it means is that 3dB is 2 times for Power but only SQRT(2) for voltage.

An example: an amplifier with 40dB gain has power gain of 1000 times but a voltage gain of only 100 times, (assuming same input and output impedance)

In this case, (what Hacknet asked about) we had a feedback of 3dB which means that the power AND voltage gain decrease by 3dB i.e. SQRT(2) ~= 1.4142 times but a power gain decrease of 2 times.

Regards Hans