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2000\09\19@103045 by

Hello all, I am trying to find three characteristics of the LF351 op-amp.

Ao = Low Frequency Gain (in dB or V/V)
Fb (some call it Fp) = Corner Frequency (frequency where gain is -3 dB)
Ft = frequency at which the gain is 1 (0 dB)

I know that Ao * Fb = Ft

and from the datasheet on http://www.national.com I _BELIEVE_ that Ao = 100,000 V/V (100 dB) and Ft = 4 Mhz (4 *10^6 Hz)  but when I plug all the numbers in... I get a corner frequency of 40 Hz!  There is no way that is a correct corner frequency.  Any help would be greatly appreciated.

--

I think that F3dB = Ft divided by *closed loop* gain. In your formula it
looks like you're using open loop gain.

The minimum stable (uncompensated) gain for an LF351 is 88 which gives you
an F3dB of just over 45kHz, which is the right order of magnitude.

Hope this is right, it's a while since I last used that formula.

{Original Message removed}
I understand that some normal op-amps could exhibit a corner frequency of
10Hz or lower. The 741 was supposed to be 10Hz, though I never checked in
data. That roll-off would only be seen if the amp was operated open loop. I
guess that means I can't amplify 10uV to 1V in one stage.

I haven't got the hang of gain-bandwidth-product yet, current mode seems to
have changed all the rules too.

On the subject of op-amps does anyone know a unity gain stable power op-amp?

Oliver.

--

Oliver,

The Gain-Bandwidth Product is simply Voltage Gain (AV) times the
Bandwidth (BW) of the amplifier.  Lets assume the Op-Amp in
question has a unity gain bandwidth of 10 Mhz and a maximum open
loop gain of 10000.  And lets further say that you want an
amplifier gain of 200. Then the BW of the amplifier is 10Mhz divided
by 200 for a BW of 50Khz.  Or, lets say you need a BW from this
amplifier of 500Khz and an Av of 40.  In this case, you'll either
have to cascade two opamps of this type or use a different type to
achieve the required gain.  This is because if you need 500Khz BW
and you divide 10Mhz by 500Khz, you get 20, so you'll have to get
the other 2X amplification from a second opamp.

Hope this very simple, quick and dirty (and incomplete) explanation
helps you out.

Regards,

Jim

On Tue, 19 September 2000, Oliver Broad wrote:

{Quote hidden}

jimjpes.com

--

>
>I haven't got the hang of gain-bandwidth-product yet, current mode seems to
>have changed all the rules too.

GBP = GAIN * Bandwidth (hz)

An op-amp with a GBP of 1,000,000 will give you a gain of 1000 with -3db at
1000 hz

>On the subject of op-amps does anyone know a unity gain stable power op-amp?

LM-12 :)

How much power did you really want?
--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--

Sounds reasonable to me you won't actually be running open loop with a gain
of 100000 will you? . At a gain of 100 the corner frequency will be about
40kHz.
Richard P

Hello all, I am trying to find three characteristics of the LF351 op-amp.

Ao = Low Frequency Gain (in dB or V/V)
Fb (some call it Fp) = Corner Frequency (frequency where gain is -3 dB)
Ft = frequency at which the gain is 1 (0 dB)

I know that Ao * Fb = Ft

and from the datasheet on http://www.national.com I _BELIEVE_ that Ao = 100,000
V/V (100 dB) and Ft = 4 Mhz (4 *10^6 Hz)  but when I plug all the numbers
in... I get a corner frequency of 40 Hz!  There is no way that is a correct
corner frequency.  Any help would be greatly appreciated.

--

--

>>
and from the datasheet on http://www.national.com I _BELIEVE_ that Ao = 100,000 V/V
(100 dB) and Ft = 4 Mhz (4 *10^6 Hz)  but when I plug all the numbers in...
I get a corner frequency of 40 Hz!  There is no way that is a correct corner
frequency.
<<

Why do you say that?  That is the frequency at which signals start to no
longer look like DC to the device.  It is quite common for this to be at a
"low" frequency for high gain devices.

*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olincognivis.com, http://www.cognivis.com

--

> Hello all, I am trying to find three characteristics of the LF351 op-amp.
>
> Ao = Low Frequency Gain (in dB or V/V)
> Fb (some call it Fp) = Corner Frequency (frequency where gain is -3 dB)
> Ft = frequency at which the gain is 1 (0 dB)
>
> I know that Ao * Fb = Ft
>
> and from the datasheet on http://www.national.com I _BELIEVE_ that Ao =
> 100,000 V/V (100 dB) and Ft = 4 Mhz (4 *10^6 Hz)  but when I plug
> all the numbers in... I get a corner frequency of 40 Hz!  There
> is no way that is a correct corner frequency.  Any help would be
> greatly appreciated.

That sounds about right, remember since you are using a finite Ao the
amplification of 100dB only lasts for a few Hz. The corner freq will change
depending on what gain you plan to use the Op Amp at, you are currently
figuring out the corner freq at open loop gain. Hope this helped, TTYL

--

>On the subject of op-amps does anyone know a unity gain stable power op-amp?

All the op-ams specced to be stable at unity gain are stable at unity
gain. Or rather, all op-amps not specced to be compensated for unity gain
may not be stable at unity gain. In reality, most op-amps compensated for
unity gain may be stable, but not all of them. ;-) And even those that are
can be turned into transmitters by adding a suitably long piece of wire to
the output, and almost nothing else.

LM324 is a cheap one that is compensated (but beware some clones which
exhibit a lot of noise (>8mV pk-pk and up to 2-3MHz) in this mode). Just
buy a good make. Also, many op-amps compensated for unity gain strongly
object to capacitors connected directly between the output and ground.
They tend to oscillate like this, or induce noise (in despite of the cap).

Peter

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From: David VanHorn <dvanhornCEDAR.NET>
> LM-12 :)

The thought had crossed my mind. Maybe four of them giving two bridged
outputs for a special application. Mostly I only want maybe 10 or 20 watts
intermittant, I really don't want to build a buffer output stage as it would
have no overload protection and ideally I want short to ground and short to
negative rail protection and thermal shutdown.

L165 works great once I get round that minimum gain restriction but it's a
pain.

Oliver.

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Like that!

Thanks but I did say power, I guess I should have put TO220 style power to
be precise. Two or three power devices available gain-of-ten stable ie L165.
Numerous nearly identical audio parts with same pinout and opamps in all but
name, minimum gain not stated as such but can be inferred to be 15 or 20.
The ones I've tried work well in DC applications.

Having to use them only in inverting configurations is a pain as is not
being easily able to limit BW.

Following your comments I also note many devices 'object' to 50 ohm cable.

Oliver.

{Original Message removed}

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