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Amplifier Design with a large banwidth

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KSHATRIYA

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Hi all,
I want to design an amplifier which would amplify a sinusoidal signal from (0-300mV) at least 100 times. My frequency band is (10khz-1Mhz). Suppose my maximum input change would be around 20mV so I take a slew of around 2.3V/us. For choosing the right amplifier I see the frequency response curve of the amplifier and check whether in my operating frequency, will it be able to give me that gain. So I finally zeroed on OP27. I am using OP27 as a basic non-inverting amplifier (single one with a gain of 10) but I am unable to get any amplification rather I am getting the input DC Voltage with a little loss. I have not kept my resistor between Bal1 and Bal pins. Is my approach correct or I am missing something.

thank you for reading

Regards
Kshatriya
 

Please post your schematic. Another thing for you to analyze the problem is, 10kHz to 1Mhz is quite large range. and amplifying 300mV signal by 100 times gives voltage of 3V. Check the out put stage voltage range.
Right amplifier selection is totally depends upon the input signal nature and its application. Elaborate more of your problem.
Anyways irrespective of applications I shall suggest 3 methods,
1) Use single amplifier with gain 100 and cutoff frequency at >1MHz. Will lead to UGB of around >10Mhz.
2) Use two amplifier stage with each stage gain of 10 and UGB can easily achieved more than 10MHz.
*Case 2 will give you better response and need not to search for specific OPAMP.
3) If you have differential signal , Instrumentation amplifier is best method for many applications.

- - - Updated - - -

Case 1 shows slewing problem most of the time.
 
I don't understand your calculations:
KSHATRIYA said:
I want to design an amplifier which would amplify a sinusoidal signal from (0-300mV) at least 100 times.
Click to expand...
If you amplify 300mV 100 times, the output will be 30V. That is too much for a normal opamp.

My frequency band is (10khz-1Mhz). Suppose my maximum input change would be around 20mV so I take a slew of around 2.3V/us.
Click to expand...
2.3V/us is only enough for an output of 260mV RMS at 1 MHz.

I am unable to get any amplification rather I am getting the input DC Voltage with a little loss.
Click to expand...
It sounds like there is a problem with the biasing. Can you show your circuit diagram?
 
you need 40dB at 1MHz means unity gain bandwidth >100MHz. usually too much for common. I suggest use 2 to 3 stage amplifier to do that job
 
varunkant2k said:
Please post your schematic. Another thing for you to analyze the problem is, 10kHz to 1Mhz is quite large range. and amplifying 300mV signal by 100 times gives voltage of 3V. Check the out put stage voltage range.
Right amplifier selection is totally depends upon the input signal nature and its application. Elaborate more of your problem.
Anyways irrespective of applications I shall suggest 3 methods,
1) Use single amplifier with gain 100 and cutoff frequency at >1MHz. Will lead to UGB of around >10Mhz.
2) Use two amplifier stage with each stage gain of 10 and UGB can easily achieved more than 10MHz.
*Case 2 will give you better response and need not to search for specific OPAMP.
3) If you have differential signal , Instrumentation amplifier is best method for many applications.

- - - Updated - - -

Case 1 shows slewing problem most of the time.
Click to expand...

Thank you for the answer, Indeed I am using an amplifier OP27 from the Gain, Phase Shift vs. Frequency plot (pg no -10 ) in the datasheet, I can use this OPAMP at 1MHz for a gain between 15-19db. Am I getting it right. I only want to have a gain of 10 as you pointed out, so can I use this OPAMP?. Is my understanding of the graph correct? as you pointed out GBW product of this OPAMP is around 8Mhz, so I can use this OPAMP for amplifying the signal which will be in the range of (10kHz-1MHz). I am also attaching the datasheet. As said Idon'st need any input offset so I am not using the pins that are for them. Also I am giving an input of around 11V

- - - Updated - - -

Can you please elaborate on Unity Gain banwidth and Slew rate calculation for a given sinusoidal signal. I assume that to find slew rate we need to Vin*2*3.14.f, if I take worst case and assume that I am having a 1Mhz signal . Then I get a slew rate of 1.456V/us. Is it right or as I have given Vp-p as 232mV is there any change in calculation. At present i am only trying with a DC signal and I get the same problem as above. Am I perceiving something incorrectly

Thank you all for the answers

Regards
Kshatriya
 

Attachments

  • OP27.pdf
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KSHATRIYA said:
I assume that to find slew rate we need to Vin*2*3.14.f
Click to expand...
That is the slew rate at the input.
Slew rate at the output = Vout*2*3.14.f
The opamp's datasheet shows the maximum slew rate at the output.
 

godfreyl said:
That is the slew rate at the input.
Slew rate at the output = Vout*2*3.14.f
The opamp's datasheet shows the maximum slew rate at the output.
Click to expand...

Thank you for the information. I am using the following circuit. Anything wrong in it.
 

Attachments

  • im.jpg
    im.jpg
    36 KB · Views: 104

KSHATRIYA said:
Thank you for the information. I am using the following circuit. Anything wrong in it.
Click to expand...

Yes, something wrong.
If you are using single supply (instead of dual supply) you have to apply correct input biasing. This has been discussed in this forum several times.
 

KSHATRIYA said:
Thank you for the information. I am using the following circuit. Anything wrong in it.
Click to expand...
Yes. The OP27 is not designed to operate with a single supply. And, even if it did, if your input is a standard AC signal that goes above and below ground you could only amplify the positive half of the signal since there is no negative voltage available.

To operate with a single supply you need to bias the input and output signals at 1/2 the supply voltage such as shown here.
 
I'm still not sure exactly what the requirements are, but OP27 is not really "high speed" by today's standards - for example, there are several opamps with slew rates over 1000V/us.
 

Thank you all, But at present I have not applied any AC voltage to it, as according to the figure it is Vin =0.1V Dc volatage. Then also I am getting supply Vdd at the output.
@Crutschow
Thank you for the schematic, I mean , I am using it as a general single ended voltage amplifier, then also do I need a negative supply and in datasheet also its not mentioned that OP27 works only on dual supply , But thanks to you , I came to know that it only works on Dual supply, thaks for the info I was able to get the output.Now I will try with sinusoidal signal

Output Slew
I still don't know how to deal with that or how shall I deal with in my design. Has anyone used OP27 in such designs?

Thank you all, Its been a great learning experience.

Regards
Kshatriya
 
Last edited:

Hi all,
With dual supply I was able to play around with DC voltages, but when I am giving the AC supply voltage, I am getting it amplified but the frequency is not the same.
I shall also make the circuit according to the schematic given by crutschow and verify.amp.jpg
 

KSHATRIYA said:
Hi all,
With dual supply I was able to play around with DC voltages, but when I am giving the AC supply voltage, I am getting it amplified but the frequency is not the same.
Click to expand...

This is impossible. It must be a measurement error or something similar.
 

No I have verified it Oscilloscope, I will do it again and post the results here.
 

10k and 1k in feed back is not correct values. need to increase by 10 times.
 

KSHATRIYA said:
Hi all,
With dual supply I was able to play around with DC voltages, but when I am giving the AC supply voltage, I am getting it amplified but the frequency is not the same.
Click to expand...

Hi KSHATRIYA,

I hope you do not mean "ac supply voltage" but instead ac signal input voltage, OK?
 

To KSHATRIYA: if you have peak voltage of 300mV, then its not possible to get a multiplication factor of 100. The reason is simple. even though in calculation you get 100 or more, the opamp which you use just goes to saturation. You have to use an opamp which has the voltage rating of 60-60V to get the desired characteristics if at all you want to do it with opamp. if not i wil give a better suggestion.. ! Why not go for transistor based model? you can get 100+ times multiplication under condition that you dont reach saturation! so chosing Vcc is very important.
 

msharmavikram said:
To KSHATRIYA: if you have peak voltage of 300mV, then its not possible to get a multiplication factor of 100. T
Click to expand...

That's correct - however, the circuit as shown in post#7 has a gain of +11. This should work in principle.
 

Thank you all. I was able to get the requisite amplification over the required frequency range. Thank you for clearing my queries. Actually there is one more query that I want to discuss here. I have used the circuit which I have posted in the figure. i will be using the same circuit to amplify a small signal which is output of a coil / transformer . Actually I was able to get the amplification when I connected it directly, but what in excess I can do to provide better input coupling to transformers output. Any suggestions ?
 

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