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Old 19th April 2005, 10:38 PM   #1
arte is offline arte  Sweden
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Default Simple AMP Comments.

Hello,

I am a Electronics student, its my first year so i am not that good. I have been experimenting with pSpice trying to design a simple amplifier.
I have made one that seems to work in pSpice but its not that good, linearity is not perfect and efficeny is about 40%.

I dont even know if it works at all.

Anyway here is the schematics, any comments would be nice!

Click the image to open in full size.
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Old 20th April 2005, 12:53 AM   #2
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Hi Arte

For a start lose R17 completely and lift R16 off pin 1 (output pin) of the IC and instead connect it to the output point of the amp (junction of the MOSFETs).

Your MOSFETs are now in the chip/amp feedback loop which should cut the THD considerably. Of course this is a very simple basic amplifier but instructional. You may get almost 5W from it.
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Old 20th April 2005, 01:09 AM   #3
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HI THere buddy good looking basic amp
omit r17 and if you like increase the input z of the amp r9 to a higher value you like and dont forget this in turn will affect gain readjust the feedback r of the opamp to reset gain to whatever value you like
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Old 20th April 2005, 08:42 AM   #4
arte is offline arte  Sweden
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Quote:
Originally posted by amplifierguru
Hi Arte

For a start lose R17 completely and lift R16 off pin 1 (output pin) of the IC and instead connect it to the output point of the amp (junction of the MOSFETs).

Your MOSFETs are now in the chip/amp feedback loop which should cut the THD considerably. Of course this is a very simple basic amplifier but instructional. You may get almost 5W from it.


AAA good! Didnt think about that.

A few questions:

How do can i calculate the Power of the amplifier?
Is it bad to use an IC instead of tranistors,the circuit gets SO much simpler?
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Old 20th April 2005, 10:00 AM   #5
arte is offline arte  Sweden
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I forgot, the voltage over the diods is not constant, it differs between 2.3v to aprox 3.7. Shouldnt this be a constant voltage that is just above MOSFETS Gate Threshold?
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Old 21st April 2005, 12:12 AM   #6
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Arte,

power is = Output Voltage (rms) squared/ load impedance

This is very compromised due to limited drive current from the 25K resistors and the Vgs losses of the MOSFETS limiting available swing. You could bootstrap or use current sources and add two drive transistors with inverted MOSFETS to reduce output stage losses. Or just raise the supplies.
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Old 21st April 2005, 12:32 AM   #7
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Hi arte didnt i tell you it is nice looking basic amp, you mentioned that the
diode drops vary, i didnt put them there buddy you did and you knew that
didnt you, i have invented a super-constant bias technique that varies only
within the range of +-40uV you cant use that with your basic amp can you?
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Old 22nd April 2005, 07:36 AM   #8
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arte,

Your sim software should have an idealised current source so replace the 25Ks with these and note any difference! Change MOSFETs for easier drive ones = note the difference.

You can learn a lot from this before you burn one component or breathe any lead!
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Old 22nd April 2005, 10:48 AM   #9
ilimzn is offline ilimzn  Croatia
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And as far as I know, there is no commonly available OP-amp that works off +-35V power supply.
Do not rely just on the simulator - they most often do not have any limit values to their models (unless the models support failure analysis, which is rare), so when you select a component, you need to read it's data sheet, at least as far as normal operating conditions for the part.
The voltage across the diodes varies with current, as well as temperature. The threshold voltage for the MOSFETs also varies with temperature. All of these parameters vary from one to the other example of real life diodes and MOSFETs even if they are of the same type. Because of this you commonly need a means of adjusting the bias voltage (that which you use your diodes for).
Also, although the MOSFETs need very little current into their gates (practicly zero) under static conditions, it is not so under dynamic conditions because gates have a capacitance, which is not very small either, and is not linear. The simplest way to fix this on your amp is to put a capacitor in parallel with each of the diode strings.
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Old 22nd April 2005, 11:29 AM   #10
moamps is offline moamps  Croatia
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Quote:
Originally posted by ilimzn
And as far as I know, there is no commonly available OP-amp that works off +-35V power supply.
OPA445 (9, Farnell)
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