npn booster Zin

[seanm]

I built the Gus Smalley's NPN boost to try as a high gain boost. I made some mods to boost the gain. The gain pot was increased to 10k. The resistor from collector to +9 was increased from 10k to 47k and the 100k resistor increased to 220k. This kept the biasing right with an MPSA18.

This worked well. Then I decided to calculate the gain. According to the text the Zin should be about hfe x unbypassed gain pot (i.e. min gain). So I was expecting about 700 * 10k or 7M! Actually I was realistically expecting maybe a meg.

I put a 100k resistor in front of the input cap and connected R.G. Keen's quick and dirty oscillator at about 105mv. I measured V2 from osc. side of resistor to ground and measured V1 from boost side to ground. Then:

Iin = (V2 - V1) / R1   and Zin = V1/Iin

V2=106mv   V1=78mv  R1=99k

I ended up with about 276k... much less than I expected. Any ideas of what went wrong?

[Khas Evets]

In calculating the input impedance, did you factor in the 10K + 47K series resistors to ground? You also have to consider the bootstrapped 22uF cap, which increases impedance.

[seanm]

In calculating the input impedance, did you factor in the 10K + 47K series resistors to ground? You also have to consider the bootstrapped 22uF cap, which increases impedance.

No I didn't. I considered the oscillator and the booster "black boxes" and just used the resistor to calculate the impedance. I also played around with the bootstrapped cap and its value did not seem to matter.

Is there something wrong with my method?

[R.G.]

Nope, your method is OK.

I did some simulator runs. Simulators have to be watched very carefully, because they live is a simulated perfect world, and what they tell you doesn't always match this real one. But for some things, they're just ducky.

One "just ducky" thing is figuring bias points, impedances, etc.

I modelled the NPN boost and the sim comes out with a Zin of about 300K with the gain pot at halfway - not far from your result.

I messed with it a bit. The bootstrap resistor, the 10K from the base to the bias string 100K/47K looked funny to me because all of the bootstrap circuits I've seen use 100K and higher. I whipped in a 100K, the zin went up, and the bias point was about the same. I upped it to 1M and zin went up again to 1.6M and the bias on the collector stayed under 4.7V.

I think if you change that 10K to a 1M, you get a much higher input impedance.

[seanm]

I whipped in a 100K, the zin went up, and the bias point was about the same. I upped it to 1M and zin went up again to 1.6M and the bias on the collector stayed under 4.7V.

I think if you change that 10K to a 1M, you get a much higher input impedance.

Thanks for the info. I will try 1M tonight! Did you simulate with the original values or my modded ones?

I also tried playing with the bootstrap resistor. It did not seem to affect the bias (this was before I checked the Zin). So that seems to match the simulator.

I *did* try playing with the bootstrap cap. Changing it from 22uF to 1uF did not seem to affect impedance or perceived gain. So I left it at 1uF.

If this works, it may become the new front end to my Mr Smooth.

[seanm]

Ok, I could not get the 1M to bias properly, so here it is with 470k. I split up the pot to up the minimum impedance. It now ranges from ~1.3M to ~2M.

Unlike my earlier comment, as the bootstrap resistor goes above 100k, it starts affecting the bias. Also, at higher values of the resistor, *reducing* the bootstrap cap also seems to increase input impedance.

If you change the transistor, you will probably need to giggle with the bias. I tried a 2N3904 and the bias was way too high.

Now I just need to increase the gain.....