# Op Amp Exam Questions

Discussion in 'Homework Help' started by Cesc_1984, Dec 28, 2009.

1. ### Cesc_1984 Thread Starter New Member

Dec 28, 2009
4
0
Hi,
im new to the forum and am studying electrical energy engineering. A minor module we study is 741 operational amplifiers. I have an exam on the 11th of Jan and need help with the following 2 questions. Ive searched google and this forum and cant find an answer that will do. The questions are from past exam papers and are as follows:

1 - What is meant by maximum power dissipation and drop-out voltage when associated with integrated circuit voltage regulators. A voltage regulator has a drop out voltage of 2V and a maximum power dissipation of 2W. Determine the maximum and minimum voltage which can be applied to a 12V regulator when conducting a current of 100mA.

2 - Sketch to scale the asymptotic and actual bode frequency response for a direct coupled amplifier with a cutoff frequency of 100kHz and a passband gain of 10. Why is the amplitude expressed in dB's? At what frequency is the gain 0dB's.

Thats it, sorry if they seem really easy, its my first year and I find electronics harder than electrical circuit theory. Any help will be greatly appreciated.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
63
Maximum power dissipation is the maximum power a voltage regulator can dissipate without to fail. It equals the current through and the voltage across the regulator (Vin-Vout).

Drop out voltage is the minimum voltage across the regulator (Vin-Vout) as for the regulator to work within its specifications.

Try to calculate the maximum and minimum voltage now.

3. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The asymptotic is the one which is approximated by straight lines. The actual is the curved one at the cut-off frequency.

The basic reason for expressing the amplitude in dBs is beacuse when you derive the bode plot of a system, the poles and zeros individual responses are added which is easier rather than multpiply them if the amplitude was not expressed in dBs.

If the amplifier has only a single pole, then the gain will drop by 20dBs/decade.

4. ### Cesc_1984 Thread Starter New Member

Dec 28, 2009
4
0
Thanks for the help,
I worked it out to be min voltage - 14V & max voltage - 32V.
I came across one more question which is the last one I had difficulties with.

A 1V RMS sinusodial signal is applied to a non inverting op amp having a voltage gain of 3. The op amp has a gain-bandwidth product of 10 MHz and a slew rate of 1V/us. At what frequency will slew rate distortion occur.

I used the formula: Fmax = SR/2*Pi*Vp,
where Vp is the peak value of the wave.

I converted the slew rate to V/s which equals 1000000 and converted the RMS to peak which equals 1.414. I then subbed everything into the equation and got an answer of 112.523 kHz.

Im not sure if this is the frequency at which distortion would occur. And why am I supplied with a gain in the question if it is not used. Or am I going about it the wrong way.

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You have to use the gain because it is the peak of the output signal you have to put in the equation and not the input.

6. ### Cesc_1984 Thread Starter New Member

Dec 28, 2009
4
0
37.536 kHz.

An amp has a differential gain of 10 and a CMRR of 80 dB. It is fed from a balanced source which provides a differential signal of 100mV and a noise signal of 2V. Calculate the total output voltage.

80 = 20log(10/Acom)
4 = log(10/Acom)
10^4 = 10/Acom
Acom = .001

Acom = Vo/Vcom
.001 = Vo/2
.001 x 2 = Vo
Vo = 2mV

In this question i didnt use the differential voltage. How come also the GBW is not used but provided in the slew rate question.

7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
It deosn't mean that you have to use all the information given in a question.

8. ### mik3 Senior Member

Feb 4, 2008
4,846
63
You have to use the differential voltage and gain to find the output voltage caused by it and add it to the common mode voltage.