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low input impedance amplifier

M

MooseFET

Jan 1, 1970
0
Please suggest very low input impedance amplifiers, preferably
operational ampliers.



If this is for DC operation (ASCII art):

---------/\/\------------------------------
! !
! ----!!------ !
IN ----+---!+\ ! ! !
! >--+---/\/\--+--!-\ ! !
--!-/ ! ! >------+----------
! ! GND---!+/
+---!!---
!
\
/
\
!
GND

Op-amps = TL072
Resistors = 1.1M
Capacitors = 50u 25V Mylar
 
E

Eeyore

Jan 1, 1970
0
Please suggest very low input impedance amplifiers, preferably
operational ampliers.

Please suggest your application.


Operational; amplifiers by definition all have HIGH input impedance. I strongly
suspect you don't want/need an op-amp.

Graham
 
E

Eeyore

Jan 1, 1970
0
MooseFET said:
"krishman...wrote:


If this is for DC operation (ASCII art):

---------/\/\------------------------------
! !
! ----!!------ !
IN ----+---!+\ ! ! !
! >--+---/\/\--+--!-\ ! !
--!-/ ! ! >------+----------
! ! GND---!+/
+---!!---
!
\
/
\
!
GND

What on earth is that ?

Op-amps = TL072
Resistors = 1.1M
Capacitors = 50u 25V Mylar

50u mylar ??????

Graham
 
T

Tom Bruhns

Jan 1, 1970
0
Please suggest very low input impedance amplifiers, preferably
operational ampliers.


Please suggest more design design requirements, preferably related to
the application.
 
K

Kevin Aylward

Jan 1, 1970
0
Eeyore said:
Please suggest your application.


Operational; amplifiers by definition all have HIGH input impedance.
I strongly suspect you don't want/need an op-amp.

Graham

err....whats wrong with a umm... feedback summing amp...for low freq, opamp
methods will beat them all at tend to zero ohms
 
K

Kevin G. Rhoads

Jan 1, 1970
0
Please suggest very low input impedance amplifiers, preferably
operational ampliers.

Virtual earth (a.k.a., virtual ground) current to voltage converter:
 
K

Kevin G. Rhoads

Jan 1, 1970
0
---------/\/\---- !
! !
! !
IN ----+---!-\ !
! >-----------!
--!+/
!
!
GND

Resistance sets the I to V ratio, input appears "grounded" over ranges
of operation where OpAmp can servo it.
 
W

whit3rd

Jan 1, 1970
0
Please suggest very low input impedance amplifiers, preferably
operational ampliers.

Operational amplifiers are inverting and have high gain (at DC
as well as at signal frequencies). Almost all idealizations
of op amps add 'fully-differential' and 'high input impedance'
and 'low output impedance' to this.

Norton amplifiers (current-input-differencing) like LM3900
have low input impedance BUT that goes with requirement to
ensure with suitable external components that the input doesn't
get too much current (and burn up) or go negative (and
saturate/shut down the amplifier).

Some op amps (like uA741) have 'input offset' pins that
amount to a low-impedance input option.

And an inverting-gain connected op amp (ground the (+) pin
and use R1 from signal to (-), and R2 from output to (-) ) has
the input impedance equal to R1; it can be anything you want.
 
M

MooseFET

Jan 1, 1970
0
What on earth is that ?

Work out the input impaedance at Dc and you will see.
50u mylar ??????

Use 2 25uF ones in parallel if you want. The values given were based
on the assumption it was a home work question.
 
Dear all,
I mean something like transimpedance amplifier..where the input is
current and ouput is voltage...such amplifier invariably has low input
impedance...please suggest any commercial amplifier for it.
thanks
kristo
 
W

Winfield

Jan 1, 1970
0
kristo said:
by commercial ..i mean opamp or other amplifier avialble in IC package.

kristo, we all just use a feedback resistor (plus capacitor) and
chose a suitable opamp for the problem. For example, a JFET
opamp for low currents and high transresistance values, or a
high-speed bipolar opamp for high current and low resistance.
For really high speeds, in the GHz territory, such as for fibre-
optic data receivers, its useful to have the feedback resistor
integrated with a specialized opamp, and in fact its most useful
to have the detector and optical connectors in the package too.
 
M

MooseFET

Jan 1, 1970
0
bu commercial ..i mean opamp or other amplifier aviable in IC package.


You haven't stated any frequency requirements. The circuit I posted
way back in this thread actually would work but only for low
frequencies.

Go to www.linear.com and download a copy of "SwitcherCad-III". It is
a very good general purpose spice program. You can use it to verify
your design when you get it done.

The details of how you do a low input impedance amplifier vary a lot
with the frequency requirements but the general idea is usually the
same. You always have an op-amp circuit that greatly amplifies any
voltage that may appear on the input terminal and a feedback resistor
that passes a large current for any given voltage.

Consider this example condition:


100R
1uV input ----+-------/\/\---------
! !
! !\ !
-------! >----------
!/ Gain = -1 million

The output of the Gain stage will be -1V so 10mA will go through the
100R. As far as the 1uV input is concerned the input resistance is:

R(in) = 1uV / 10mA = 0.0001R

For low frequencies, op-amps like the TL072 will provide the gain you
need. You can use a two stage amplifier with two op-amps. (One would
have to be inverting and the other not)
 
Y

YD

Jan 1, 1970
0
Late at night, by candle light, "[email protected]"
bu commercial ..i mean opamp or other amplifier aviable in IC package.

If it's DC or low frequency AC at reasonably high level just about any
old thing like a 741 might do. Whatzit you need it for?

- YD.
 
M

MooseFET

Jan 1, 1970
0
I want the amplifier to have low input impedance till 50MHz.


How low is low?


At 50MHz, you are looking at a fairly fast op-amp being needed.

You may want to look at parts like the LT1398. You aren't going to be
able to have much gain in the amplifier and make it stable.
 
J

John Popelish

Jan 1, 1970
0
MooseFET said:
How low is low?


At 50MHz, you are looking at a fairly fast op-amp being needed.

You may want to look at parts like the LT1398. You aren't going to be
able to have much gain in the amplifier and make it stable.

This might be a good application for a current feedback op
amp like this one:
http://www.onsemi.com/pub/Collateral/NCS2510.PDF
 
F

Fred Bartoli

Jan 1, 1970
0
John Popelish a écrit :
This might be a good application for a current feedback op amp like this
one:
http://www.onsemi.com/pub/Collateral/NCS2510.PDF

Don't know if it's necessary to get that far, depending on what krishman
needs:
CFB opamps have inherently low minus input impedance, typically between
50-100R.
If that's not enough, then this opamp will achieve 'only' 11R at 50MHz.

This figure could be improved with a generously biased discrete CB pair
at the input stage of a composite opamp.

But if the low frequency isn't DC and really low impedance is mandatory,
a small current transformer might be more appropriate, or an opamp-CT
hybrid if he has to get down to DC.

As usual it'd be better if we knew what the OP wants to do in the end.
 
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