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Topic: Opamp setup doubt (Read 12399 times) previous topic - next topic

Grumpy_Mike

Quote
Does that sound right?
No.
The transistor is all shorted out.
The base goes to the op-amp output no where else.
The 4K7 is missing
You have changed the configuration from the origional diagram.

Marciokoko

#46
Jul 13, 2016, 05:14 pm Last Edit: Jul 13, 2016, 05:14 pm by Marciokoko Reason: image
Ok here is a better one...


Grumpy_Mike

R4 and R5 are in parallel. Remove one of them.

Marciokoko

#48
Jul 13, 2016, 07:00 pm Last Edit: Jul 13, 2016, 07:01 pm by Marciokoko Reason: image
Like this?



Being in parallel is wrong because why?  The effective resistance is reduced?

Grumpy_Mike

#49
Jul 13, 2016, 10:20 pm Last Edit: Jul 13, 2016, 10:21 pm by Grumpy_Mike
Quote
Being in parallel is wrong because why?
Because it is a waste of a resistor. Two identical resistors in parallel is the same as having just one resistor at half the value.

In that last diagram you seem to have shorted out the speaker.

Marciokoko

you mean shorted because there is nothing between it and Ground?

But then how can I not have it be in parallel with the Emitter resistor-to-ground you said I should add?

Grumpy_Mike

Quote
But then how can I not have it be in parallel with the Emitter resistor-to-ground you said I should add?
Are you telling me that in reply #46 R4 was the speaker? If so why was it given a value of 4K7? I thought we had established it was 32R.

Marciokoko

#52
Jul 13, 2016, 11:09 pm Last Edit: Jul 13, 2016, 11:09 pm by Marciokoko Reason: image
Oh!  Yeah, I just dont know how to draw a speaker in ltspice.  I dont have that component.  Ok, so you want to see the speaker as a 32Ω load, which is what it is.  But you said you wanted a:

"4K7 R from emitter to ground as well as the load. "

I understood that as adding a 4K7 R between Emitter and Ground and between Speaker and Ground.

So like this:


Grumpy_Mike

Yes that is better. The R6 resistor is so that things still work correctly when the speaker is disconnected.

Marciokoko

Well I can hear some rattling sound out of the speaker when I move a resistor which seems loose (R3 mic+toVcc+).  Probably just noise.  So to the schematic...Im assuming it can be calculated, what the voltage and current flow should be at each point in the circuit, right?

Will LTSpice just tell me and then I can check those values against what I see on the board?

Grumpy_Mike

Quote
Im assuming it can be calculated, what the voltage and current flow should be at each point in the circuit, right?
Yes

Quote
Will LTSpice just tell me and then I can check those values against what I see on the board?
Yes

Marciokoko

#56
Jul 14, 2016, 04:36 pm Last Edit: Jul 15, 2016, 01:14 am by Marciokoko Reason: image
Here are my LTSpice results:

Vin (Signal) = 30mV
Vp ~ 0.18mV
Vn ~ 80mV
Vout ~ 8.95V
Vaudioout ~ 8.05V

BJT = 2.5mA
R1 & R5 (negative feedback Rs) ~ 79uA
R2 ~ 0 mA
R3 ~ 1.3mA
Speaker ~ 250mA
R6 ~ 0mA
C1: Current is in the fA.

Here is the image:

I can see the 30mV signal amplified to 8.95V and the current bumped from 2.5mA to 250mA.  Seems logical.

But Im only getting 0.48V at Vout in the circuit :-(

polymorph

4K7 is shorthand for 4.7K. Saves typing a dot. Either would work. 4.7K ohm resistors are common.
It isn't shorthand, it is because if you print or write 4.7k on a schematic, it doesn't take much to either mistake the dot for a bit of smudge, or it might really be 47k but you mistake a bit of smudge for a dot.

4k7 is much harder to mistake for another value.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

polymorph

That is a terrible way to drive a speaker. You will only see the positive going cycle on the speaker, as half of the waveform is clipped. That circuit only has the ability to source current into the speaker, not sink it.

You want something more like this:


By the way, is the center connection between the two 9V batteries connected to circuit ground?
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

ChrisTenone

It isn't shorthand, it is because if you print or write 4.7k on a schematic, it doesn't take much to either mistake the dot for a bit of smudge, or it might really be 47k but you mistake a bit of smudge for a dot.

4k7 is much harder to mistake for another value.
That's the history. Digital files don't smudge.
What, I need to say something else too?

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