another power amplifier- lm675 questionsplease help

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nza0104

Joined Oct 17, 2010
4
Hi!

I'm constructing the "Non-inverting amplifier" in page 1 of the attached datasheet. In my circuit I replace the 20k ohm resistor with a 1k ohm resistor since I actually need a voltage gain of 2.
1. If the motor is set to turn forward, the voltage gain is correct, but if I set the motor to turn backward/ reverse, I do not get the correct voltage gain. What is my mistake? From my observation, if I increase the current from my power supply (I use +-8V), it will also affect the voltage gain. What is the correct current that I should set at my power supply unit?
2. I have two motors whose power needed to be amplified. I am using 2 units of LM675. I have only one set of power supply unit, so I just connect the first LM675's pin5 to the second LM675's pin5 and then to the VCC. Then I also connect the first LM675's pin3 to the second LM675's pin3 and then to the VCC. Is this right? I do not get the correct voltage gain with this connection. What is the correct way of connecting the amplifiers, since I only have one set of power supply unit?
3. What is the effect of 0.1mircroF capacitors at VCC and VEE? Is it ok not to use any capacitors at all at VCC and VEE?

Thanks.
 

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SgtWookie

Joined Jul 17, 2007
22,221
1. If the motor is set to turn forward, the voltage gain is correct, but if I set the motor to turn backward/ reverse, I do not get the correct voltage gain. What is my mistake?
You've miswired the Vee connection to Vcc instead of ground/power return.
From my observation, if I increase the current from my power supply (I use +-8V), it will also affect the voltage gain. What is the correct current that I should set at my power supply unit?
That depends on how much current your motors require with the load you have on them.
2. I have two motors whose power needed to be amplified.
The underlined portion of the above statement does not make sense.
I am using 2 units of LM675. I have only one set of power supply unit, so I just connect the first LM675's pin5 to the second LM675's pin5 and then to the VCC. Then I also connect the first LM675's pin3 to the second LM675's pin3 and then to the VCC. Is this right?
No, it is not. The pin 5 part is OK, but both pin 3's need to be connected to ground/supply return.
I do not get the correct voltage gain with this connection. What is the correct way of connecting the amplifiers, since I only have one set of power supply unit?
Explained above.
3. What is the effect of 0.1mircroF capacitors at VCC and VEE? Is it ok not to use any capacitors at all at VCC and VEE?
Without those capacitors, the amplifiers may not be stable.

You really need to post your circuit as it exists, along with what you are using to supply the input signal.
 

Audioguru

Joined Dec 20, 2007
11,249
I'm constructing the "Non-inverting amplifier" in page 1 of the attached datasheet. In my circuit I replace the 20k ohm resistor with a 1k ohm resistor since I actually need a voltage gain of 2.
Then your amplifier is oscillating.
The "Application Hints" section in the datasheet says that it is not stable if its gain is less than 10.

You can add a resistor to make the voltage gain about 20 and make the "noise gain" 2. The datasheet shows how on page 7.
 

Audioguru

Joined Dec 20, 2007
11,249
I'm constructing the "Non-inverting amplifier" in page 1 of the attached datasheet. In my circuit I replace the 20k ohm resistor with a 1k ohm resistor since I actually need a voltage gain of 2.
Then your amplifier is oscillating.
The "Application Hints" section in the datasheet says that it is not stable if its gain is less than 10.

You can add a resistor and maybe a capacitor to make the "noise gain" higher than 10 and make the voltage gain 2. The datasheet shows how on page 7.
 
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