# Need guidance on this homework problem

Joined Feb 23, 2020
15
I have a problem in my ET212 class that I truly have no idea on where to start, I have the circuit design, but am clueless when it comes to the values that I need to use. I am not asking for answers, mainly just guidance on where I am supposed to start, or the steps i need to do to find my answers.
Project details are 250mV peak input voltage, 20-20k Hz range, 8.4W power output 8 ohm load. I just have no idea where to start with analyzing and figuring out what capacitors or resistors I need to use. I know that I (probably) need to start with getting the voltage gain set for each transistor, but the darlington pair i dont understand well. The circuit design is below, any help is greatly appreciated. The 1k ohm and 1uF values are placeholders, as well as the VCC value.

#### schmitt trigger

Joined Jul 12, 2010
482
This is clearly a discrete transistor audio amplifier.
Di you have to operate Class A?

Joined Feb 23, 2020
15
This is clearly a discrete transistor audio amplifier.
Di you have to operate Class A?
The description doesnt say exactly, just that we cant use OP amps, I would assume so though as that is all that we have discussed so far.

#### schmitt trigger

Joined Jul 12, 2010
482
First thing you have to calculate:

what is the voltage gain required to develop 8.4 watts across 8 ohms, with an input voltage of 250 mV?

#### BobTPH

Joined Jun 5, 2013
4,299
Where did you get that circuit? Was it part of the assignment?

You say you don’t know what values to use, but they are all on the schematic. Do you think you can change the values and meet the requirements of the assignment?

How about this for starters: What voltage swing, peak to peak, is required to get 8.4W in an 8 Ohm speaker?

Bob

Joined Feb 23, 2020
15
roughly 32.8 on the gain to get the needed 8.4W from 250mV. Is that right?

Joined Feb 23, 2020
15
Where did you get that circuit? Was it part of the assignment?

You say you don’t know what values to use, but they are all on the schematic. Do you think you can change the values and meet the requirements of the assignment?

How about this for starters: What voltage swing, peak to peak, is required to get 8.4W in an 8 Ohm speaker?

Bob

Hey
Not entirely sure what voltage swing is, and yes it is from an assignment, we need to figure out the values to get the needed output power with the supplied input voltage. The resistor and capacitor values in the circuit right now are just place holders to design the circuit. Need to find the actual values needed.

Joined Feb 23, 2020
15
First thing you have to calculate:

what is the voltage gain required to develop 8.4 watts across 8 ohms, with an input voltage of 250 mV?
roughly 32.8 on the gain to get the needed 8.4W from 250mV. Is that right?

#### Bernard

Joined Aug 7, 2008
5,788
From the top of my head, first 2 stages a gain of about 10, C1 much too low. 1uF @ 1kHx about 100 ohms.

#### BobTPH

Joined Jun 5, 2013
4,299
Well , your professor is a dolt, because a curcuit like that cannot practically produce 8.4W.

By voltage swing, I mean the peak to peak voltage of a sine wave that would produce the 8.4W power. You clearly know this since you correctly calculated the gain.

Bob

#### WBahn

Joined Mar 31, 2012
26,398
Well , your professor is a dolt, because a curcuit like that cannot practically produce 8.4W.
I haven't run any numbers to see how feasible it is, but practicality probably doesn't play into it. It's a design exercise covering specific topics and is further limited in that it can't utilize concepts from topics to be covered later. It's very likely that one of the points of the assignment is to highlight the issues that would make using this approach impractical specifically as a segue into later topics to motivate more practical approaches.

This of the typical progression of simpler topics -- people learn resistive voltage dividers early on and it isn't uncommon to have an exercise in which such a divider is used as a power supply for some circuit, the primary intent being to drive home why resistive voltage dividers make lousy power supplies. This is then often followed by zener diode based supplies and again an exercise is used to illustrate that they are only suitable as supplies in pretty narrow situations. Then later various forms of linear regulators get introduced. The notion of switched-mode power supplies is a topic that doesn't come along until much, much later. I wouldn't consider the professor that has the design exercise involving a resistive voltage divider to be a dolt because they didn't use a SMPS.

Joined Feb 23, 2020
15
Well , your professor is a dolt, because a curcuit like that cannot practically produce 8.4W.

By voltage swing, I mean the peak to peak voltage of a sine wave that would produce the 8.4W power. You clearly know this since you correctly calculated the gain.

Bob
The needed input voltage and output power were choosable, the options were 100, 150, and 250mV for input voltage and 3.2 W, 5.6 W, 7.1 W, 8.4 W, 11.5 W, and 16.3 W for the output power. It is a group project and we each have to make a amplifier with different selections. I just chose the 8.4W and 250mV randomly.

#### WBahn

Joined Mar 31, 2012
26,398
The needed input voltage and output power were choosable, the options were 100, 150, and 250mV for input voltage and 3.2 W, 5.6 W, 7.1 W, 8.4 W, 11.5 W, and 16.3 W for the output power. It is a group project and we each have to make a amplifier with different selections. I just chose the 8.4W and 250mV randomly.
Are you going to actually build, test, and demonstrate your amplifier, or is it just design and simulate?

Joined Feb 23, 2020
15
Are you going to actually build, test, and demonstrate your amplifier, or is it just design and simulate?
Design and simulate only

#### BobTPH

Joined Jun 5, 2013
4,299
If you were to actually build it, it would become a lesson on heat sinks.

Bob

Joined Feb 23, 2020
15
If you were to actually build it, it would become a lesson on heat sinks.

Bob
Simulation only thankfully

#### WBahn

Joined Mar 31, 2012
26,398
Design and simulate only
That's what I suspected and it's a real shame. You would have learned SO much more if you actually had to build it. Smoking transistors is a much better teacher than a number displayed on a schematic as the result of a simulation.

Joined Feb 23, 2020
15
That's what I suspected and it's a real shame. You would have learned SO much more if you actually had to build it. Smoking transistors is a much better teacher than a number displayed on a schematic as the result of a simulation.
This is actually the final project of this course, and it is an online class as well. The hardware portion is very limited sadly. Currently I am trying to figure out the middle stage of the amp, i think I have gotten the power amplifier part finished(hopefully) and just need the other two voltage amplifier portions. If you want to glance at my Power amp portion and see if anything sticks out to you as wrong, that would be great. The main things that I am not sure about are the input voltage to this part of the circuit and the value of C1.

Edit: I forgot to say that I have decided to change the needed output power to be 3.2W instead of the original 8.4.

#### Audioguru again

Joined Oct 21, 2019
4,129
Since the student knows nothing about the voltage swing (peak-to-peak) of an audio amplifier and knows nothing about how to design a simple single transistor circuit and connect them in series then he can't do this project until he is taught about electronics.

Another homework assignment where the teacher does not teach anything but expects the students to find a forum where an expert will do it (for free?) for the students.

Oh, now the output Darlington emitter-follower is shown with an 8V supply and its base is biased at 4V. Then its emitter cannot go lower than about 4V minus its 1.3V base-emitter voltage drop which is 2.7V peak. Then its RMS output is voltage is 2.7V x 0.707= 1.91V RMS. Then the maximum output power in the 8 ohm speaker is 1.91V squared/8= 0.46W.
Its "peak power" is 0.92W and its extremely distorted squarewave power (phoney power) is 1.84W.

But he wants 3.2W that ain't gonna happen. 3.2W in an 8 ohm speaker is 5.2V RMS which is 14.7V peak-to-peak. Then the supply for this Darlington transistor must be 1.3V higher which is 16V minimum.

Oh, I forgot. The 8 ohm resistor R3 will not produce enough negative voltage swing so the supply must be 14.7V/4= about 3.7V higher which is 19.7V minimum.

#### Alec_t

Joined Sep 17, 2013
12,477
In your post #1 circuit the base and emitter of Q1 are shorted together. Is that intentional or a typo?