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amp notes

i changed the circuit back to 1K input impedance, 100K feedback in
first stage. the second stage has 1K + 100nF, and 100K feedback, and I
have no idea why this works: less noise, and it seems to have a good
response in the intended range..

maybe because most sounds have a 1/f response? i don't know... it
responds well to whistles, which is nice.

this is a 10 kHz pole... so it's basicly set up as a differentiator?
maybe because i have GBW rolloff this works? i'm puzzled.

i tried with a LM358N and it gives a lot more noise.

i tried TL072CN and it gives too much bandwidth! so, i use a
compensated integrator with 1K/100n in the source and 220K/4.7n in the
feedback section. looks like this is final enough.. maybe beef up the
amp just a tiny bit more..

PARTS LIST:

2 x 220K
1 x 100K
2 x 10K
3 x 1K

2 x 15pF
1 x 4.7nF
2 x 100nF
2 x 10uF

1 x 10MHz
1 x 18F2620
1 x TL072CN
1 x LED(red)
2 x 6 PIN HEADER
                                           C2 4.7nF
                                        /-----||----\
                                        |           |
   SPK            Rg 220K               |  R2 220K  |
    o          /-/\/\/\--\              o--/\/\/\---o
    |          |  __     |              |  __       |
    |   Rs 1K  | |  \    |       R1 1K  | |  \      |
    o--/\/\/\--o-| - \___o--||--/\/\/\--o-| - \_____o LINE
|\  @        ____| + /     C1 100nF     __| + /
| ||@       |    |__/                   | |__/
|/  @       o---------------------------/
    |       |
   === Cs   |
    |  10uF |
    |       |
    |   Cn  |
    |  10uF |
o---o---||--o
|           |
o--/\/\/\/--o---/\/\/\/--o
GND        2.5V         5V


First stage gives 220 x amplification.

TL072 (TI version, i'm using ST version) has a GBW of 3 MHz, with 220
x amplification this gives rolloff at 13 kHz. so for the first stage
i'm good.

Second stage is a band pass filter with 22 x amplification:

G . . . . ._________
          /.       .\
         / .       . \
        /  .       .  \
         1/t2     1/t1

t1 = R1 C1 = 100us  -> 10kHz
t2 = R2 C2 = 1000us ->  1kHz


because f1 > f2 the gain is not R2/R1 but R2/R1 * f1/f2.

a bit quirky, but it works.. maybe i should try with exchanging the
time constants so f2 > f1.

looks like these changes keep the transfer function the same, with a =
sqrt(10)

R2 -> 1/a R2
C2 -> 1/a C2
R1 -> a R1
C1 -> a C1

so, there's a reason to do it like i did! the capacitors are
smaller. so where's the trade-off? maybe noise due to large resistors?
however, when f2 > f1 the gain is independent of the capacitors.

let's make this a bit more intuitive. what happens when C1 is made 10x
larger, so f1 = 1kHz, and C2 is made 10x smallr, so f2 = 10kHz? the
gain is now 10x more, so then the gain can be reduced by making R2
10x smaller, which again requires C2 to be 10x larger. so the net
effect is:

R1 -> R1
C1 -> 10 C1
C2 -> C2
R2 -> 1/10 R2

this gives a 1uF capacitor. so alternatively R1 can be made 10x
larger, which requires C2 to be made 10x smaller. giving 10K and 470pF
respectively. (EDIT: this is what i did. works fine).

makes more sense now. so is there a ciruit that has independent
frequency and gain?

hmm.. i just tried the LM358N again, and it gives good results
also. guess the TL022 was just too low bandwidth? yup. 0.5 Mhz. hmm
the LM358N is only 1MHz ?







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