general question about feedback, input impedance, and noise

In a feedback amplifier where there is an input resistor, say 2k for example, a 47k resistor from gate to ground, and a feedback resistor of 100K on the same input (inverting amp), does the 2k in series affect noise performance? Also, how do you calculate the input impedance of such a stage (JFET input)?
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> does the 2k in series affect noise performance?

Yes.

2K pure resistance has about 0.8 microvolts of noise over the audio band. This is generally lower than the active device noise, unless you have been VERY careful in your amplifier noise analysis. It is over 120dB below "hi-fi line level", and it is hard to find a source that is as quiet as a 2K resistor.

Common phono pickups have self-resistance of a few K, so another 2K adds a little noise (when the needle is not in a groove!). However such pickups mostly can't pull a 2K load, being intended to work with 47K.

Dynamic microphones have low output and self-resistance near 200 ohms. I assure you that a cheap PA head with 2K resistors in the input diff-amp will have enough excess hiss to notice. Not when the band is playing, but it was a poor choice for a children's poetry reading in a live room. I have another board with 470 ohm inverting inputs, and nobody complained about its noise, but mikes do sound different with ~500Ω load than with the >2K loads most mikes are designed for and most boards provide.

> how do you calculate the input impedance of such a stage (JFET input)?

If gain and feedback is "high", then the input impedance is 2K.

If gain is low, much lower than 100K/2K= ~25, so feedback is nearly zero, the input is 2K+(100K||47K)= ~32K.

If gain is around 100, the 100K will act like ~50K to ground, so input is ~27K. If gain is like 500, the 100K looks like 10K, so we have ~6K. Open loop gain needs to be near 2,500 to get that 100K to look like 2K, and get total input impedance down toward 4K. You need gain of 25,000 to get to within 10% of the pure 2K.

> resistor to the ground (47k) does not affect input impedance and gain.

It does, a little. I admit and agree that it makes about no practical difference for these values. It also adds noise: depending on the external source impedance, maybe less than 1dB and certainly less than the noise most sources (full of 2K and 50K resistors) already have. I do wonder why JFETs need 47K gate-leak resistor.

> What if the input impedance of the LTP is 50k

If it is JFET: then something is mighty wrong.

Even for high-current BJTs, the naked input resistance is increased by feedback. For any well-defined high-NFB design, it will be "infinite enough". If low-NFB, you need to do some elaborate calculations, throw it at SPICE, or whomp-up a breadboard (the best way to get the real answer).
 
> What if the input impedance of the LTP is 50k

If it is JFET: then something is mighty wrong.

Even for high-current BJTs, the naked input resistance is increased by feedback. For any well-defined high-NFB design, it will be "infinite enough". If low-NFB, you need to do some elaborate calculations, throw it at SPICE, or whomp-up a breadboard (the best way to get the real answer). [/B]

That is what I thought when the statement was made that the input impedance with a 2k in series and 25k to gnd will have an input impedance of 2k... I thought it would be the lower of the 25k or LTP input impedance not 2k which is in series...
 
Thank you for the replies.
Would I be correct to assume that I never see any phono stages with feedback and an inverting amplifier because the series input resistor degrades noise performance, and also makes for too low of an input impedance? I'm speaking of MM here, not moving coil.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> I never see any phono stages with feedback and an inverting amplifier because the series input resistor degrades noise performance, and also makes for too low of an input impedance?

I've seen the idea used; you have the disadvantages figured out.

The noise may not be a killer fault once you drop the needle on vinyl. Surface noise is generally far higher than the noise of a modern device. In fact the disk levels and cartridge sensitivity were all balanced to work well with the noise of a vacuum tube, say 2 microvolts good and 5 uV typical. A hot-chisel cut on acetate carefully pressed to pure vinyl will have surface noise a little higher than tube noise. When we go to <1uV transistors, we get better test-bench specs but not better listening.

A perfect 47K resistor will have 3uV noise. More than the best tube, less than many popular tube phono amps.

Going to lower input impedances gets confusing because the impedance of the pickup is not well specified. That is, until you get to zero input impedance. Then the electrical response is flat (per your feedback impedance). However the custom has been to allow the pickup inductance and cable capacitance to resonate at the top of the audio band, limited by the Standardized 47K resistance. Some cartridges have a few dB electrical bump to compensate a falling mechanical response; others with excess mechanical response may be balanced for a slump; some (old Grado) just don't resonate much with normal capacitance and 47K. Many fancy preamps have switchable C and R loading so you can fiddle the top octave to spec or to taste. With a zero-Z input you lose this fine-trim on the top, and may need a EQ trim in the electrical system.