# How to Calculate LM386 gain?

Discussion in 'Homework Help' started by disney_snoopy, Mar 5, 2009.

1. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
Hi all,

Can any1 teach me how to calculate the gain=20?
izit the Vin is from the pin3 of the IC's or measure the Vin right after my microphone?

And the Vout is measure from the pin5 or measure at the speaker???

Plz comment on this...

thank you.

2. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Gain is controlled between pins 1 and 8, if both are left disconnected, the default gain is 20 (Output voltage is 20x input voltage, or 26dB)

The datasheet has the info on changing the gain, as well as an internal schematic that shows how a capacitor between pins 1 and 8 change the gain, as well as several example circuits.

http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/8/LM386.shtml

The 20x voltage gain is between the Input (pins 2 and 3), and VOUT (pin 5).

3. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
huh...
i think this website should band this batateam...
As he is sending a porn photo/movie to us... maybe it is a virus in the website...

4. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
thatoneguy,

but i have a problem in getting this gain.
i measure the pin3=0.6V and pin5=3V(maximum volt i can get)
so my gain=5 instead of 20...

So what should i do le?

Last edited by a moderator: Mar 5, 2009
5. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
thatoneguy,

i measure pin3=0.6V and pin5=3V(max voltage that i can get)
from here my gain=5 instead of 20
what should i do to solve the problem so that i can get gain=20?

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Are you measuring a waveform with an oscilloscope, or are you getting the voltage from a multimeter measurement of DC levels?

7. ### Audioguru New Member

Dec 20, 2007
9,411
896
Pin 3 is at 0VDC so he must be using a multimeter set to read AC volts.
A multimeter reads the 50Hz or 60Hz mains frequency sine-wave. It is not accurate at higher frequencies with any other waveform.

8. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
I use multimeter to measure the volt but i set it to DCvolts.
However, i change the setting of multimeter to ACvolts; it gives me 0.045 for Vin while my Vout is 0.9V.
Hence, my gain was accurate to 20.
Is that correct that change the multimeter to ACvolts?
Plz comment...

9. ### Audioguru New Member

Dec 20, 2007
9,411
896
The input and output volts of an audio amplifier are AC, not DC.

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Have you tried connecting a speaker to the circuit?

That's a good test to see if more gain is needed for the input source level.

Multimeter AC Range isn't useful, as you are getting either "mean" or RMS voltage, depending on meter, and usually only at low frequencies, under 200Hz. DC Range won't give you any useful information, since audio is AC, you are only seeing the bias output from a single ended supply.

If you need DC gain, Operational Amplifiers such as the TL082 are designed for high voltage gain, but low current ability, which, among other factors, makes them not very good as an audio power amplifier.

11. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
ya i do connect speaker after a capacitor of the LM386 output pin5.
but since the case that u said using multimeter ACvolts only measure frequency below 200Hz, then what tools should i use to measure the frequency above 200Hz?

if using oscilloscope, then how to measure???
Can explain more details?

Thank You.

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Oscilloscope probe from ground to speaker terminal, while driving speaker.

If your power supply is only 6 Volts, attach a 0.25V peak-peak sinewave signal generator to the input. Look at the oscilloscope, you should see a tall sinewave with peak-peak voltage of around 5. If you increase the output of the signal generator above 0.3V, you will notice the sinewave "Flattens out" at the top and bottom, looking more like a square wave in extreme cases.

This is the amplifier clipping, as the gain is increasing the signal extends past what the power rails will allow (0V and 6V).

If you do not have a signal generator, whistle into a microphone from a distance, change distance to change level. This produces a surprisingly useful sine-ish waveform.

13. ### Audioguru New Member

Dec 20, 2007
9,411
896
The datasheet for the LM386 shows its output is a max of 4V p-p when its supply is only 6V and the speaker is 8 ohms. then its output power is only 0.25W at a horrible-sounding 10% distortion or a whopping 0.2W at clipping. Flea-power.

14. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
Granted, the LM386 definitely isn't a "Power Amplifier" as one would normally define it. At the same time, there isn't a simpler to use, stable (term used loosely) IC amplifier to compare with it.

For use as a signal tracer/low power, they are useful. If you require flat response from 10Hz to 30kHz with low THD+N, this isn't the IC you are looking for.

Look at this nice waveform when the output is loaded. Input is in blue.
BTW: That waveform sounds very icky .

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15. ### Audioguru New Member

Dec 20, 2007
9,411
896
Oh yeah?
Its frequency response is from almost DC to 300kHz.
Its distortion at clipping is only 0.2%.

Your simulation software sees no supply bypass capacitor. Then of course the waveform is messed up.

16. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
I believe I screwed up the LM386 Model... It didn't exist, and I left some stuff tacked on.

Do you have a correct model for an LM386?

Code ( (Unknown Language)):
1.
2.
3. ##################  Model Data Report  ##################
4.
5. **************** Operational Amplifier **********************
6. *                                                           *
7. *              O* LM386 subcircuit model follows:
8. ************************************original* IC pins:     2   3   7   1   8   5   6   4
9. * IC pins:     1   2   3   4   5   6   7   8
10. *              |   |   |   |   |   |   |   |
11. .subckt LM386  g1  inn inp gnd out  vs byp g8
12. ************************************original*.subckt LM386 inn inp byp  g1  g8 out  vs gnd
13.
14. * input emitter-follower buffers:
15.
16. q1 gnd inn 10011 ddpnp
17. r1 inn gnd 50k
18. q2 gnd inp 10012 ddpnp
19. r2 inp gnd 50k
20.
21. * differential input stage, gain-setting
22. * resistors, and internal feedback resistor:
23.
24. q3 10013 10011 10008 ddpnp
25. q4 10014 10012 g1 ddpnp
26. r3 vs byp 15k
27. r4 byp 10008 15k
28. r5 10008 g8 150
29. r6 g8 g1 1.35k
30. r7 g1 out 15k
31.
32. * input stage current mirror:
33.
34. q5 10013 10013 gnd ddnpn
35. q6 10014 10013 gnd ddnpn
36.
37. * voltage gain stage & rolloff cap:
38.
39. q7 10017 10014 gnd ddnpn
40. c1 10014 10017 15pf
41.
42. * current mirror source for gain stage:
43.
44. i1 10002 vs dc 5m
45. q8 10004 10002 vs ddpnp
46. q9 10002 10002 vs ddpnp
47.
48. * Sziklai-connected push-pull output stage:
49.
50. q10 10018 10017 out ddpnp
51. q11 10004 10004 10009 ddnpn 100
52. q12 10009 10009 10017 ddnpn 100
53. q13 vs 10004 out ddnpn 100
54. q14 out 10018 gnd ddnpn 100
55.
56. * generic transistor models generated
57. * with MicroSim's PARTs utility, using
58. * default parameters except Bf:
59.
60. .model ddnpn NPN(Is=10f Xti=3 Eg=1.11 Vaf=100
61. + Bf=400 Ise=0 Ne=1.5 Ikf=0 Nk=.5 Xtb=1.5 Var=100
62. + Br=1 Isc=0 Nc=2 Ikr=0 Rc=0 Cjc=2p Mjc=.3333
63. + Vjc=.75 Fc=.5 Cje=5p Mje=.3333 Vje=.75 Tr=10n
64. + Tf=1n Itf=1 Xtf=0 Vtf=10)
65.
66. .model ddpnp PNP(Is=10f Xti=3 Eg=1.11 Vaf=100
67. + Bf=200 Ise=0 Ne=1.5 Ikf=0 Nk=.5 Xtb=1.5 Var=100
68. + Br=1 Isc=0 Nc=2 Ikr=0 Rc=0 Cjc=2p Mjc=.3333
69. + Vjc=.75 Fc=.5 Cje=5p Mje=.3333 Vje=.75 Tr=10n
70. + Tf=1n Itf=1 Xtf=0 Vtf=10)
71.
72. .ends
73. *----------end of subcircuit model-----------perational Amplifier Model                  *
74. *            Interactive Image Technologies                 *
75. *          SPICE MODEL MAKER TOOL  Version 1.1              *
77. *                                                           *
78. *************************************************************
79.
80. * Date: Tuesday, March 10, 2009
81. * Model Name generic_uA741
82.
83. * INPUT VALUES:
84.
85. * Page 1
86.
87. * General
88. * Model Name   generic_uA741
89. * Identifier               0
90.
91. * Input
92. * Input Capacitance                = 1.400e+000 pF
93. * Input Offset Current                = 2.000e+001 nA
94. * Input Bias Current                = 8.000e+001 nA
95. * Input Offset Voltage                = 1.000e+000 mV
96. * Common-mode Input Resistance            = 2.000e+000 GOhm
97. * Differential-mode Input Resistance        = 2.000e+000 MOhm
98. * Common-mode Rejection Ratio            = 9.000e+001 dB
99. * Voltage Gain                    = 1.060e+002 dB
100.
101. * Page 2
102. * Gain-frequency curve poles and Zero
103. * Pole 1                        = 5.000e+000 Hz
104. * Pole 2                        = 2.000e+000 MHz
105. * Pole 3                        = 2.000e+001 MHz
106. * Pole 4                        = 1.000e+002 MHz
107. * Zero 3                        = 5.000e+000 MHz
108.
109. * Page 3
110.
111. * Output
112. * Slew Rate (non-inverting mode)             = 5.000e-001 V/us
113. * Slew Rate (inverting mode)             = 5.000e-001 V/us
114. * Output Resistance                 = 7.500e+001 Ohm
115. * Maximum Source Output Current             = 2.500e+001 mA
116. * Maximum Sink Output Current             = 2.500e+001 mA
117.
118. * OP-AMP Model
119. .SUBCKT generic_uA741 1 5 9 11 14
120. * terminal 1: invert input
121. * terminal 5: non-invert input
122. * terminal 9: positive power source
123. * terminal 11: negative power source
124. * terminal 14: output
125. .MODEL DMOD D (N=0.001)
126. r1 3 0 2.000e+009
127. r3 3 0 2.000e+009
128. c3 3 5 1.400e-012
129. r2 3 5 2.000e+006
130. vc 3 1 1.000e-003
131. i1 3 0 9.000e-008
132. i2 4 0 7.000e-008
133. r6 13 0 1.000e+003
134. c2 13 0 7.958e-011
135. g4 13 0 3 4 1.000e-003
136. r7 17 18 1.000e+004
137. c4 17 18 3.183e-012
138. r8 18 0 3.333e+003
139. e1 17 0 13 0 4.000e+000
140. r9 19 0 1.000e+003
141. c5 19 0 1.592e-012
142. g5 19 0 3 4 1.000e-003
143. r4 0 6 1.000e+003
144. c1 0 7 3.183e-005
145. vb 6 7 dc=0
146. g1 0 6 19 0 1.995e+002
147. g2 0 6 poly(2) 3 0 4 0 0 3.155e-003 3.155e-003
148. d7 7 9 DMOD
149. d3 7 8 DMOD
150. d4 8 9 DMOD
151. d8 11 7 DMOD
152. d5 10 7 DMOD
153. d6 11 10 DMOD
154. f1 8 9 poly(1) vb -1.592e+001 1
155. f2 11 10 poly(1) vb -1.592e+001 -1
156. va 12 14 dc 0
157. d9 14 15 DMOD
158. d10 15 9 DMOD
159. d12 11 16 DMOD
160. d11 16 14 DMOD
161. f3 15 9 poly(1) va -2.500e-002 1
162. f4 11 16 poly(1) va -2.500e-002 -1
163. g3 0 12 7 0 1.333e-002
164. r5 0 12 7.500e+001
165. rshunt1 9 0 1e6
166. rshunt2 11 0 1e6
167. .ENDS
168.
169. ============= Model template =================
170. x%p %tGAININ %tINPUT- %tINPUT+ %tGND %tOUT %tVS+ %tBYPASS %tGAINOUT %m
171.

17. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
sigh... i read it late... i was in college but i forgotten to bring my circuit to college... will test it tomolo.

by the way, maybe i know what software simulation that have the LM386 and is download free?

18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
718
You can get LTSpice or a variety of other SPICE like programs for free on the net.

I believe the one from orcad.com is used by a few people here.

As far as the LM386 Model, I found a couple variants on the net, but they aren't working so good, as you can see above...

I'm sure somebody here has a proper model for one.

19. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Here's the model I have. I think it works.
Code ( (Unknown Language)):
1. q1 gnd inn 10011 ddpnp
2. r1 inn gnd 50k
3. q2 gnd inp 10012 ddpnp
4. r2 inp gnd 50k
5.
6.
7. * differential input stage, gain-setting
8. * resistors, and internal feedback resistor:
9.
10.
11. q3 10013 10011 10008 ddpnp
12. q4 10014 10012 g1 ddpnp
13. r3 vs byp 15k
14. r4 byp 10008 15k
15. r5 10008 g8 150
16. r6 g8 g1 1.35k
17. r7 g1 out 15k
18.
19.
20. * input stage current mirror:
21.
22.
23. q5 10013 10013 gnd ddnpn
24. q6 10014 10013 gnd ddnpn
25.
26.
27. * voltage gain stage & rolloff cap:
28.
29.
30. q7 10017 10014 gnd ddnpn
31. c1 10014 10017 15pf
32.
33.
34. * current mirror source for gain stage:
35.
36.
37. i1 10002 vs dc 5m
38. q8 10004 10002 vs ddpnp
39. q9 10002 10002 vs ddpnp
40.
41.
42. * Sziklai-connected push-pull output stage:
43.
44.
45. q10 10018 10017 out ddpnp
46. q11 10004 10004 10009 ddnpn 100
47. q12 10009 10009 10017 ddnpn 100
48. q13 vs 10004 out ddnpn 100
49. q14 out 10018 gnd ddnpn 100
50.
51.
52. * generic transistor models generated
53. * with MicroSim's PARTs utility, using
54. * default parameters except Bf:
55.
56.
57. .model ddnpn NPN(Is=10f Xti=3 Eg=1.11 Vaf=100
58. + Bf=400 Ise=0 Ne=1.5 Ikf=0 Nk=.5 Xtb=1.5 Var=100
59. + Br=1 Isc=0 Nc=2 Ikr=0 Rc=0 Cjc=2p Mjc=.3333
60. + Vjc=.75 Fc=.5 Cje=5p Mje=.3333 Vje=.75 Tr=10n
61. + Tf=1n Itf=1 Xtf=0 Vtf=10)
62.
63.
64. .model ddpnp PNP(Is=10f Xti=3 Eg=1.11 Vaf=100
65. + Bf=200 Ise=0 Ne=1.5 Ikf=0 Nk=.5 Xtb=1.5 Var=100
66. + Br=1 Isc=0 Nc=2 Ikr=0 Rc=0 Cjc=2p Mjc=.3333
67. + Vjc=.75 Fc=.5 Cje=5p Mje=.3333 Vje=.75 Tr=10n
68. + Tf=1n Itf=1 Xtf=0 Vtf=10)
69.
70.
71. .ends
72. *----------end of subcircuit model-----------
73.
74.
75.

20. ### disney_snoopy Thread Starter Member

Feb 19, 2009
28
0
Hi Thatoneguy,

I have replace the sine wave generator from the microphone and my probe was connect to the speaker. And i plug in the 0.25V from sine wave generator. However, my output was 3.02V from the oscilloscope instead of 5V that u said. Another point is my supply voltage is 6V from 4 AA battery.

Can u tell me why my gain is only 15 compare than what u said gain=20(5V at output).