# Electronic discussion

1. Jan 13, 2017

### physicist 12345

• Misplaced homework moved from technical forum, hence the Homework Template is missing.
at firs i apologize if this is not the place for this but really i have exam after few hours and i need chech my solutions .
i have some essay questions and i want to check my ideas

1) can we use the diode as switch instead of transistor ??
2)design an op amp to amplify the dc signal
3)how to use two electronic switches to get ((rectangular)) wave shape
4) define 3 differences between
(responce curve of op amp and voltage amp)
(power amp and volt amp and op amp)

my ideas for solution
1) no because diode make no amplification
2) i will use the op amp usually as it direct connected in its stages
3) i suggest using astable multivibrartor
4) no attempt solution

2. Jan 13, 2017

1) Yes and no, generally no
2) Hmmm.. Scale yea, so technically I have to say yes. Example 0-1V signal scaled to 0-10V etc.
3) What? switch on - switch off =Rectangular wave.

4) Not enough info to even guess

3. Jan 13, 2017

### analogdesign

1. You don't need amplification to make a switch. The switches in a digital logic circuit don't amplify. The big problem with using a diode as a switch is you have to deal with the DC level shift (the diode drop, about 0.65V or so in silicon). MOS switches have no DC drop and are preferred. Diode switches are still used in very high speed sampling circuits since they can be faster than other techniques.

2. You mean design a circuit *using* op amps to amplify a DC signal? That was their initial application so yeah, they can be used for that.

4. An op amp is just a differential voltage amplifier with extremely high gain, high input resistance, and low output resistance so what are you asking? Some people use op amps to make power amps but the issue is *power* gain, not voltage gain. The simplest and easiest to under stand is an Emitter Follower (look it up). By lowering the impedance while keeping the voltage (roughly) the same, power gain is realized.

4. Jan 13, 2017

### physicist 12345

thank you
question 4 gust generally no specific cases

5. Jan 13, 2017

### physicist 12345

thank you very much but could you tell me how op amp could amplify dc ?

6. Jan 13, 2017

### davenn

7. Jan 13, 2017

### physicist 12345

8. Jan 13, 2017

### davenn

you said it is DC ... so what shape do you think it will have ?

not sure what you mean ?

it would seem you haven't watched the video yet .... do so and then come back with specific unanswered Q's
you really shouldn't have any ... Dave explains op-amps very well

Dave

9. Jan 13, 2017

### physicist 12345

iam sorry

10. Jan 13, 2017

### davenn

it's OK

so if DC is going in, what do you think is on the output of an Op-amp and what "shape" is that ?

11. Jan 13, 2017

### physicist 12345

its also dc .. i was asking how the level rise (amplified sinusoidal take not change their reference )
now watching the video :)

12. Jan 13, 2017

### davenn

yes ... so a flat line

an op-amp can be used in inverting or non inverting mode that will affect the positive and negative going half cycles compared to the input
.... they will be the same ... non-inverting .... opposite if inverting

13. Jan 13, 2017

### physicist 12345

then now i understood that ordinary op amp could amplify dc .. put as it go out flat (a t the same reference) how it then be said that they amplified
please also if you have a comment on question 4 i will be happy

14. Jan 14, 2017

### davenn

firstly, I cant comment on that ... go with analogdesign's comments until get something better

The first thing you need to learn/understand and many others also have this misconception ..... that transistors / IC's amplify .... they don't

What they actually do is to use the flow of small currents to control the flow of larger currents .... this gives the appearance of amplifying a current / voltage
They are all just called amplifiers for sake of easy explanation ..... do some reading up of transistor functions / theory

OK from minute 13:00 in the video, Dave starts to describe how it works as a practical amplifier and how the gain is calculated
The process is the same for DC or AC

Dave

15. Jan 16, 2017

### analogdesign

Far be it from me to start an argument about semantics, but based on your definition amplification at all is impossible as it would contradict the second law of thermodynamics.

While of course an amplifier puts out less energy than it gets in, and you're most certainly correct about the small current controlling the flow of larger currents (for bipolar transistors, at least), I think most engineers would agree that the *signal* is certain amplified in a transistor. Do you disagree?