Hi all -

I am currently learning about BJT power amplifier classes and FET amplifier options, after having worked only with operational amplifiers for a long time.
A few questions are not clear to me that I was hoping to get cleared up here:

1. I'm unclear about how to "design" these circuits, step-by-step. Are there standard examples that work well and explain why they work well? I don't know how certain components can be selected to change the output. I'm also not sure how much of the output is current and how much is voltage? My understanding is that operational amplifiers only really output voltage.

2. Is it generally safe to feed an operational amplifier or a commercial mixer with a standard transistor-based power amplifier?

Thanks

 

Hi,
A good place to start would be to check out this link.
https://www.allaboutcircuits.com/worksheets/

E

 

Your Question 1 implies that you are unclear about the relationship between current and voltage, so I second Eric's suggestion.

 

To get you started on op-amps, here are some theoretical things you need to think about.

An ideal op-amp is treated as a perfect black box. We don't need to know about the internals of the black box.
An ideal op-amp has the following properties:

1. Input impedance is infinite
2. Output impedance is zero
3. Voltage gain is infinite
4. Bandwidth is infinite

Now let us examine what each of these mean in theory.

1. Input Impedance
With very high input impedance, it means that the op-amp can take an input signal without disturbing the source, i.e. the op-amp takes zero current from what it is trying to amplify.

2. Output Impedance
A very low output impedance means that the op-amp can deliver very high currents to the load. This gives you some insight into the difference between a voltage amplifier and a current amplifier (or power amplifier). A power amplifier has to have very low output impedance.

3. Voltage Gain
Infinite gain means that the op-amp can amplify the smallest signal to a million volts and more. Of course, there is a physical limitation whereby the output voltage cannot exceed that provided by the power supply.

4. Bandwidth
Infinite bandwidth means that the op-amp can operate over all frequencies from 0Hz to 1000GHz and above.

In reality, there is no op-amp that can fulfill any of the above specifications. So we have to work within the limitations of real op-amps. Let us compare the above specifications of an ideal op-amp with that of a popular op-amp such as LM324.

1. Input impedance = 100MΩ
2. Output impedance = 2kΩ
3. Voltage gain = 25000
4. Bandwidth, gain falls off to 1 at 1MHz

The first thing we note is that the LM324 is not a power amp. An output impedance of 2kΩ means than it can only drive about 8mA.

The second thing we note is that the open-loop gain drops off to 1 at 1MHz. Hence if you want some appreciable voltage gain between 1 and 100, (0 to 40dB) you are limited to signal frequencies below 40kHz.

The next point is, even though the open-loop gain is 25000 (88dB), you will not design a single amplification stage with that much gain, unless you are using the op-amp as an analog comparator circuit. Stick to a gain of 100 or lower.

The voltage gain of an op-amp is controlled by applying negative feedback, i.e. a feedback resistor (Rf in the schematics shown below) is inserted from the output pin to the inverting input pin.

The differential op-amp circuit shown above is the one you should try to remember. By grounding either input resistor Rin, you can derive all the other circuits shown. The differential op-amp circuit also gives you an idea of how you can add or subtract two or more signals using an op-amp.

References:
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/introduction-operational-amplifiers/
http://www.electronics-tutorials.ws/opamp/opamp_1.html

 

The answer to question #2 is basically yes, but the devil is in WHY and HOW.

Normally the OP Amp drives the power amp, but there are times when you may want to sample the output for some reason, such as providing feedback, or monitoring etc.

 

Thanks - I hadn't seen these worksheets before - great resource!

 

Alright, I'm struggling a bit with this one detail.

If you have a basic Class A Common-Emitter Amplifier - no biasing, no frills..like the one attached from this worksheet..
How do you determine the voltage across Rc?

I know that the voltage value at the emitter is the same as the input, minus the bias.
I know I can get the current because Ohm's law means V/R = I.
Is this something that is solved with Kirchoff?

 

If you have a basic Class A Common-Emitter Amplifier - no biasing, no frills..like the one attached from this worksheet..
How do you determine the voltage across Rc?

Use common sense and Ohm's Law.
A silicon transistor has a base-emitter voltage of about 0.7V the same as a silicon diode. So if the input is 3V then the emitter is 3V - 0.7V= 2.3V which produces an emitter current of 2.3V/1k= 2.3mA. The collector current is almost the same so the 4.7k collector resistor has a voltage across it of 2.3mA x 4.7k= 10.81V.
The collector resistor connects to 15V so the collector voltage is 15V - 10.81V= 4.19V.
The beta is shown as 120 and the emitter current has the base current added which is 2.3mA/120= 19.2uA which you can subtract from the collector current if you want more accuracy but the 0.7V base-emitter voltage drop guess should be seen on the transistor's datasheet for better accuracy.

 

Knowing Vin and the supply voltage Vsup,work out
1) the emitter voltage, hence
2) the emitter current.
Within about 1% (depending on beta), collector current Ic = emitter current Ie.
From Ie and Vsup calculate Vc.