# Need guidance on this homework problem

Joined Feb 23, 2020
15
In your post #1 circuit the base and emitter of Q1 are shorted together. Is that intentional or a typo?
Yep, good catch, that is indeed a typo in the design on my behalf.

Joined Feb 23, 2020
15
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.
This is the reason that I am here, the assignments have had nothing to do with multistage amplification, and the reading doesn't help me at all really. You are right, I have no idea how to do most of this. If I fail this assignment then I will fail the class, that why I am here to hopefully learn what I am supposed to do. If what I have there wont work, then what do I need to do to make it work? I am at a loss at this point, as I need to turn something in in the next few days.

Joined Feb 23, 2020
15
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.
So would increasing the VCC to 20+ fix thhat issue? or would it take more work?

#### BobaMosfet

Joined Jul 1, 2009
2,063
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.
View attachment 199769
I'm wondering if you realize that your teacher would welcome questions from you- they don't expect you to do this in a vacuum.... if they do, get another teacher. Teachers who actually want to teach welcome students who actually want to learn.

#### Audioguru again

Joined Oct 21, 2019
5,553
I have never designed a "home heater power amplifier" (class-A) before so my calculation for its power supply voltage was low.

In my simulation, it clipped badly with a 20V supply but works pretty well with a 24V supply to produce 3.52W into 8 ohms with fairly low distortion.

#### Audioguru again

Joined Oct 21, 2019
5,553
I guess you were never taught that a transistor has a high current gain then its base current is much less than its collector current.
Which is why your base bias resistor values are WAY TOO LOW!

My base bias resistor values are too high because my SIM software does not have a model of a Darlington so I made one with two transistors that have a very high current gain.

Your new circuit has transistors with a voltage gain of about 1.5 instead of 20. Your 1uF input capacitor feeding the two 1k bias resistors has a cutoff frequency of 320Hz so it will pass only a small amount of 20Hz.

#### BobTPH

Joined Jun 5, 2013
6,280
I solved the problem (in simulation) for the original 8.4W spec. It needed 30V Vcc and it's efficiency is an amazing 6%, with the resistor and transistor in the output stage each dissipating over 50 Watts.

That is what I meant by not practical.

Bob

#### MrAl

Joined Jun 17, 2014
9,757
Hi,

Assuming you have the last stage figured out, which means you found out that the maximum power output is Vout^2/Rout, then the questions are and in this order:
1. Do you know the input and output impedance (or resistance) of a common emitter stage?
2. Do you know how to bias a common emitter transistor stage?
3. Do you know how to calculate the internal emitter resistance 're'?
4. Do you know how to calculate the AC gain of a common emitter stage?
5. Do you know how to calculate power dissipation in a transistor biased in the linear mode?

You should also be able to figure out what overall gain you need to obtain a given AC output voltage with a given input AC voltage.

If you know or can figure out or find out the answers to these questions you can design a transistor amplifier. There are some other issues too though such as capacitor reactance and distortion.

If you really are interested and really want to get into this you can do a complete nodal analysis and therefore calculate every aspect of this design with one (rather large) set of equations. Given modern math software this is an option these days and you may even be able to calculate the transient response with a little more work.

Also, it helps a lot if you are doing the work by hand to use a linear model for the transistor both for AC and for DC biasing.

Last edited:

Joined Feb 23, 2020
15
I'm wondering if you realize that your teacher would welcome questions from you- they don't expect you to do this in a vacuum.... if they do, get another teacher. Teachers who actually want to teach welcome students who actually want to learn.
Thats the thing, my professor has not responded to me in two days

Joined Feb 23, 2020
15
I guess you were never taught that a transistor has a high current gain then its base current is much less than its collector current.
Which is why your base bias resistor values are WAY TOO LOW!

My base bias resistor values are too high because my SIM software does not have a model of a Darlington so I made one with two transistors that have a very high current gain.

Your new circuit has transistors with a voltage gain of about 1.5 instead of 20. Your 1uF input capacitor feeding the two 1k bias resistors has a cutoff frequency of 320Hz so it will pass only a small amount of 20Hz.
So the 1k resistors are just placeholders while i figure out what they are supposed to be, same with the 1uF capacitors. For R5 and R8 I am not sure the proper way to find those out. Once I get the second stage worked out, I will move on to the first stage and modify the input capacitor to the appropriate value.

#### Audioguru again

Joined Oct 21, 2019
5,553
Since your teacher does not teach and I do not want to be a teacher then I am outta here. Good bye.

#### MrAl

Joined Jun 17, 2014
9,757
So the 1k resistors are just placeholders while i figure out what they are supposed to be, same with the 1uF capacitors. For R5 and R8 I am not sure the proper way to find those out. Once I get the second stage worked out, I will move on to the first stage and modify the input capacitor to the appropriate value.
Hi,

Yes that was obvious.

But have you ever biased a transistor before?
The second stage starts with the last stage input impedance which you can estimate.