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I have an amplifier, which is essentially an amplifier and a motor driver in one (link to data sheet), and I'm having an embarrasingly hard time trying to figure out what/how much it is amplifying.

I have my amplifier hooked up to an external power source, which has an amplitude of 28V, and to my motor. I have an MCU generating a PWM signal that e.g. has an amplitude of 3.3V and a dutycycle of 50%, which is fed into my amplifier.

This PWM signal, as far as I understands, is therefore responsible for switching the FETs of the amplifier from OPEN to CLOSE and vice versa, which in turn is responsible for the average amplitude (currents/volts?) going to my motor.

Now when I have this all hooked up and running, and my dutycycle and power supply is as mentioned above, it would result in 14V (=50/100 * 28V) across my motor and this then draws about 200mA from my power supply.

What is then exactly getting amplified here, and what am I supposed to calculate as gain? Is it simply supposed to be the voltage relation between the signal being generated by my MCU and my power supply (3.3V vs. 28V) or is it the current relation between the signnal from the MCU and my power supply (64mA vs. 200mA)?

Any help is much appreciated as I am completely baffled by this problem.

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Is probably not helpful to regard this as a voltage amplifer because your input is "soft" i.e. it is a duty cycle but, if the duty cycle control is (say) 0 and 5V in amplitude (standard CMOS levels) then, for a 50% duty cycle you are putting in 2.5 volts and getting out 14 volts i.e. I've just averaged input and output. If your duty cycle were 75% you have an average input voltage of 3.75V and an output of 21V i.e. a "gain" of 5.6 i.e. no different to the 50% scenario.

However, you will have substantial current amplification because your motor is taking 200mA at 50% duty and I bet the current taken by your PWM input is in the realms of micro amps.

But, after saying all of that I don't think it's helpful to regard this amplifier as having anything as straightforward as voltage or current gain.

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  • \$\begingroup\$ Oke, that clarifies at least what is amplified and what the gain is on that amplification. But if this amplifier isn't a voltage or current amplifier, what on earth is it then? \$\endgroup\$
    – JimBob
    Commented May 4, 2015 at 13:19
  • \$\begingroup\$ It's a "driver" is my best estimate of its name. Maybe "motor driver" or "power driver" even. \$\endgroup\$
    – Andy aka
    Commented May 4, 2015 at 13:22
  • \$\begingroup\$ It can't be thought of as an amplifier, as the input is a PWM signal whose amplitude is irrelevant; it's the duty cycle that controls the output voltage and the duty cycle is determine by the digital device that precedes the driver. Can you give an indication of the complete system. \$\endgroup\$
    – Chu
    Commented May 4, 2015 at 13:44
  • \$\begingroup\$ @Chu is this comment to me or to the OP? \$\endgroup\$
    – Andy aka
    Commented May 4, 2015 at 13:53
  • \$\begingroup\$ I presume this is aimed at me. @Chu, the system is as follows. A TM4C microprocessor from TI generates the PWM signal, this signal is then fed to the 2b and 1t input pins of the "amplifier" in question, and the output of the amplifier (OUT1 and OUT2) go to the DC motor. The amplifier also has the power supply connected to it, as mentioned above. \$\endgroup\$
    – JimBob
    Commented May 4, 2015 at 14:27

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