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Here I'll show you how doubling the power doubles the damping factor,let's assume both amplifiers have the same nfb factor 100,then the open loop zout for the small amplifier is 1 ohm and for the big amplifier is 0,5 ohm...Here is the formulaszout2 = 1/100=0.01 for the small amplifierzout2 = 0.5/100=0.005 for the big amplifiernow for the damping factor (df),let's assume 8 ohms zloaddf = zload/zout2 = 8/0.01=800 for the small amplifierdf=zload/zout2=8/0.005=1600 for the big amplifierwe have doubled the damping factor not from the nfb factor but from the greater damping that comes from big amplifiers...cheers
And what if the Zout for the small amplifier is .001 Ohm?Amplifier output power and damping factor have nothing to do with each other.
hi pete,i thought you were gone,glad to see you backi came to the conclusion all impedances that are in parallel give a smaller total impendancethat's ohm's law statement!!!you mention zout for the small amplifier is .001 how is this work,can you give me an exampleas to your second phrase ,is damping only regulating zload impedance variations??
It has to do with feedback.
I design nonfeedback amplifiers,thanks Pete,
It has to do with feedback. If the smaller amplifier has a higher loop gain, say, 80dB instead of the larger amp with only 40dB, then the smaller amp, with ten times higher open loop Zout than the larger amp, would have ten times smaller Zout than the larger amp after the loop is closed.It has to do with the topology of the amplifier circuit overall, not how much power is available at the output.
Loop gain is the amount of feedback available to reduce non-linearity.If your open loop gain at 100Hz is 100dB, and the closed loop gain is 20dB, you have 80dB loop gain. That 80dB loop gain will reduce non-linearity at the output by 80dB. If you have a 10 Ohm output impedance with the loop open (no feedback), closing the loop reduces that 10 Ohm by 80dB, 10/10,000 = .001 Ohm. It does the same for distortion. If you have 10% distortion with no feedback, your distortion levels with feedback will now be .001%.
How do you get so big open loop gain(100dB),do you have a technique you could share..?
Most differential input stages that are loaded with current sources will easily reach 100dB, and more, open loop gain at 100Hz.
wow thanks Pete..what a technigue...I thought open loop gain works at the voltage gain stage also adding resistors at the emmiters of the differential input stage will it reduce gain?also smaller or bigger currents of the differential input stage what will they do to gain?
Douglas Self reduces open loop gain at the differential input stage using emitter resistors to reduce distortion by equalising the currents in the differential input stage,how well does this work,picking one thing over the other ,both are there to reduce distortion...any thoughts..anyone...
A differential amplifier loaded with current sources and current mirrors has very high gain. The voltage gain stage that translates the differential current into a voltage level can be made from a variety of topologies, but the current mirror variant creates the highest gain due to the inherent high impedance presented by a current source/current mirror.Adding emitter resistors does indeed reduce the differential gain. The amount of bias current in the differential amp has a variety of effects ranging from noise levels and offsets, to what kind of input impedance the differential amplifier presents to the source driving it.
bigger currents means more gain,ohm's law...
There's nothing wrong with that approach, but it's not necessary when you have tightly matched transistors.
It depends on the behavior of the current mirrors and differential amplifiers. Higher bias currents don't necessarily mean more gain, especially as frequency increases. It's not only about gain at 100Hz, although that's where damping factor is most crucial, but the gain-bandwidth product of the circuit. Increasing bias current may decrease the bandwidth even though low frequency gain might increase.It's a balancing act for the designer to maximize those interactive characteristics for each portion of the gain stage. High frequency performance is essential to maintain very low inter modulation distortion and should never be sacrificed for damping factor, IMO.
Pete,the open loop bandwidth product gain is reduced anyway at high frequencies,it's the closed loop bandwidth product gain that matters,yes distortion will rise with less open loop bandwidth product gain at high frequencies,with any current this happens,more current into a resistor means more open loop voltage gain even for high frequencies say 20 - 20000 hz, this bandwidth is not a problem for any amplifier...
Your mention of current into a resistor is a bit off. You're not dealing with resistors, but active devices acting as current sources which have a frequency response quite a bit different than a resistor.
