Hi everyone! I saw this amplifier in another thread on this site and decided to build it to drive power to a very simple DC motor modeled by a resistor and inductor in series. I get roughly 9.6V at the output node which after some research is enough to power a simple dc motor. I'm a second year EE student. We just talked about amplifiers in class and I want to fully understand the circuit. I just finished circuit analysis 1 and am in my first microelectronics class. The lab teaching assistant tried to explain the circuit to me, but i still don't understand what's really going on. My TA did say this circuit was a bit advanced for what we're learning, but from what i understand it is a push-pull class AB for the BJTS with current being pulled from the positive node into the circuit. The diodes are supposed to provide a voltage drop of 0.7x2=1.4V which biases the two BJTS allowing them to stay "on" to push current through. the Q1 transistor is in a class A setup which is used to also drive current through to the transistors.

 

Welcome to AAC!

This is an AF (audio frequency) amplifier, intended to drive a loudspeaker.
C1 couples the AC signal to the input of the amplifier. C2 couples the AC output to the load.
Since this is an AC amplifier, it cannot drive a DC motor.

What is your question?

 

The first engineering thing to determine before you build a circuit is the characteristics of the signal (voltage, current, frequency) you want to drive the motor.

 

Thank you! Since the intent of the circuit is for audio amplification, my new question is how can I change the circuit to drive power to the inductor and resistor load? Would changing the resistor values and adding another couple capacitors allow me to drive power to the load? I went with a sinusoidal input because I wanted to be able to make use of the negative half cycle so the PNP would be more "on" than the top NPN. Taking crutschow's advice on design characteristics, would a stable DC positive source be more ideal for a motor ? I was thinking like the motor for a small remote controlled car. It would have a switch that would allow power through it, but I don't know anything on switches cause I haven't learned it yet. So, I just wanted a simplified circuit. Would it be better to use MOSFETS?

 

Audio is AC while DC is used to drive the motor for the car.

If you want a variable voltage to control the motor speed, PWM can efficiently do that.

If you want to reverse the DC polarity to reverse the motor, then a MOSFET H-bridge is a good way to do that.

 

I see thank you! I will use a DC source instead! Would it be possible to modify the current circuit to at least kind of control a small motor? I want to have more practice with BJTS as our instructor has focused the first half of our class on them. I am familiar with pulse width modulation and i know it is really efficient and already dominating the motor market, but if possible I would like to adjust this circuit to control a small motor despite it being terrible or really inefficient.

 

An amplifier takes an input signal and outputs a signal of higher amplitude. How does that relate to driving a DC motor? What is the input? What do you want to control on the motor?

 

An amplifier takes an input signal and outputs a signal of higher amplitude. How does that relate to driving a DC motor? What is the input? What do you want to control on the motor?

My thinking was if I use a AC (DC now) input and amplify it I could control the current flowing into the motor. with that, I could control the motor's rotating speed.

 

https://hades.mech.northwestern.edu/index.php/Linear_Amplifier_Motor_Driver

https://www.researchgate.net/profil...n-of-a-Dc-Driver-Circuit-Control-Dc-Motor.pdf

these are three sources that also led me to believe it was possible to do with a class AB amp.

 

"Class AB amp" is not synonymous with "audio amp".

The first link is to a simple DC unity-gain amplifier (it's amplifying current drive capability, not voltage) acting as a voltage buffer. It happens to use a Class AB output stage.

The second is a specific audio amplifier IC whose output is differential. Each output is DC-coupled output (even though the output is only characterized and spec'ed for AC operations in the audio frequency band.

The third one, based on a very brief look, has absolutely nothing to do with audio. It is a DC driver circuit.

 

"Class AB amp" is not synonymous with "audio amp".

The first link is to a simple DC unity-gain amplifier (it's amplifying current drive capability, not voltage) acting as a voltage buffer. It happens to use a Class AB output stage.

The second is a specific audio amplifier IC whose output is differential. Each output is DC-coupled output (even though the output is only characterized and spec'ed for AC operations in the audio frequency band.

The third one, based on a very brief look, has absolutely nothing to do with audio. It is a DC driver circuit.

Ahh I see. So, my thinking that I could turn the audio amp circuit into a circuit that could drive enough power to the motor was incorrect from the start. My next question is is it possible to use a class AB amp in a circuit that drive enough power to a motor? After reading up on different amplifier classes I now know a class D would be best. Could I achieve similar results with a class AB amp?

 

Class A and AB amplifiers are linear amplifiers. They are not efficient because they waste power as heat.
Class D amplifier is a switching circuit. It has high efficiency and little power is generated and wasted as heat.

 

Thanks you MrChips, WBahn, BobTPH and crutschow for commenting.
After a couple google searches I found this and will try to create this circuit in LTSPice instead. I would like to ask though is this "H-bridge" operating in a similar idea to a full wave rectifier like with diodes and are the npn/pnp pairs acting like class AB amps in a way?

 

Thanks you MrChips, WBahn, BobTPH and crutschow for commenting.
After a couple google searches I found this and will try to create this circuit in LTSPice instead. I would like to ask though is this "H-bridge" operating in a similar idea to a full wave rectifier like with diodes and are the npn/pnp pairs acting like class AB amps in a way?
View attachment 360395

No. An H-bridge functions as a motor reversing switch.
Transistors TR1 to TR4 are switches that are fully ON or fully OFF.
When TR1 and TR4 are ON, current flows through the motor in one direction. When TR2 and TR3 are ON, current flows in the reverse direction. In this circuit as it stands, there is no mechanism to vary the speed of the motor.

Edit: This is a poorly designed motor reversing control circuit. You should not reverse the direction of a motor without first bringing it to a stop. Furthermore, there is a good chance of burning out the transistors when they short out the power supply.

 

My thinking was if I use a AC (DC now) input and amplify it I could control the current flowing into the motor. with that, I could control the motor's rotating speed.

The output is AC. For a 50Hz input, the motor will go forwards for 10ms and then backwards for 10ms. It will never rotate.