After some of the email about oscillations, made some time last night to
play around with the LM380. I only have a 14-pin flavor, not the 8-pin,
but presummably there should be little difference.
**I MADE AN ERROR in the ratio's I reported in my first post on the feedback
resistor (R1) and the +IN shunt to ground (R2). (That's what I get for
trying to depend on memory). The 470K and 10K ratio's WILL NOT WORK.
Looking on my notes at home, I used 470K and 6.8K. The positive feedback
must be limited to <300; the above ratio yields 2500, which would make it
an oscillator, as a few of you noted. 6.8K is close to 10K, but in this
case, close, but no cigar. SORRY to all who tried it in vain.
Here's the closed loop gain equation from application notes AN-69:
-Av Where:
Av(closed loop) = ------------- Av=no-feedback voltage gain ~50
Av R1=feedback resistor
1- ------- (from C13 output to +IN input)
1+R1/R2 R2=resistor from +IN to ground
You can use this to see what resistors from your junk box to use. As a
test, use Av=50. The example in AN-69 calls for R1=1M and R2=15K. This
works out to Av(cl)=192. My 470K and 6.8K combination is Av(cl)=172.
In short, for about a gain of 200, the resistor from +IN to ground (R2)
should be R1 x .015 or so.
WITHOUT adding the 200 gain positive feedback mod, here's what I got
on the LM380, wired similar to the 40-9er:
GAIN PROFILE with a 9V battery
Vin Vout Av Gain (dB)
----------------------------
10mv 400mv 40 32 Where Vin=input sine wave, p-p
20 850 42 32 Vout=LM380 output, p-p with
30 1500mv 50 34 8-ohm speaker as load
40 1800 45 33 Av=voltage gain
50mv 2200mv 44 33 Gain(dB)=20logAv
60 2100 35 31 <-- Gain compression starts here
70 2000 29 29
80 2000 25 28
100mv 2000 20 26
FREQUENCY RESPONSE
Freq. Av Gain(dB)
---------------------
10Hz 31.8 30dB Frequency response measured with
20Hz 38.6 32 test Vin=22mV(p-p) in for Vout=1000mV
100Hz 45.5 33 at 1 KHz for reference.
300Hz 45.5 33
500Hz 45.5 33 Test signal applied to -INPUT,
1KHz 45.5 33 +INPUT grounded (40-9er configuration)
3KHz 54.5 35
5KHz 52.0 34 Pretty darn flat from 20Hz to 100KHz
10KHz 50.0 34
20KHz 50.0 34
100KHz 50.0 34dB
DC Current draw on the LM380 (14-pin version) at 9v was 8.5mA.
Note that with a 9v battery, the stated VOLTAGE gain of 50 is actually
quite close. I was at first afraid that was acheivable only at +12v.
After doing the above last nite, my eyelids failed closed, so I did not
add the positive feedback mod yet. I will do that ASAP and report the above
gain and frequency response data with the positive feedback incorporated.
In the meantime, stick to the two recommended feedback resistor values
above (1M/15K or 470K/6.8K) or calculate your own ratio from the
formula.
And again, sorry the one resistor ratio I posted was erroneous. And
also, do add a .001 or so across R2 (+IN to ground).
Paul NA5N
Licensed KNOWcoder
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