Electronics Forums > Finding the input resistance of an op amp?

# Finding the input resistance of an op amp?

MRW
Guest
Posts: n/a

 11-29-2006, 10:19 PM
Sorry for all the questions. I'm just curious.

In reading, Texas Instrument's Single Supply Op Amp Circuit Collection
guide, I found this attenuator circuit:
http://img360.imageshack.us/img360/8...enuatorpj3.jpg .

It says that the input resistance should be split into two: RinA and
RinB. So I figure that the input resistance would be RinA + RinB.

In general, however, how do I go about my analyses to find the input
resistance?

I get confused easily when looking at the circuits. An example is this
page: http://www.geocities.com/ferocious_1...icpreamp2.html ... It
says that the two 27k ohm resistors are in parallel. How?

The reason why I want to know about the input resistance is because
according to http://tangentsoft.net/audio/input-cap.html, the input
capacitor value will be dependent on the series resistor and the
specified cut off frequency.

Thanks!

PeteS
Guest
Posts: n/a

 11-29-2006, 10:32 PM
MRW wrote:
> Sorry for all the questions. I'm just curious.
>
> In reading, Texas Instrument's Single Supply Op Amp Circuit Collection
> guide, I found this attenuator circuit:
> http://img360.imageshack.us/img360/8...enuatorpj3.jpg .
>
> It says that the input resistance should be split into two: RinA and
> RinB. So I figure that the input resistance would be RinA + RinB.
>
> In general, however, how do I go about my analyses to find the input
> resistance?
>
> I get confused easily when looking at the circuits. An example is this
> page: http://www.geocities.com/ferocious_1...icpreamp2.html ... It
> says that the two 27k ohm resistors are in parallel. How?
>
> The reason why I want to know about the input resistance is because
> according to http://tangentsoft.net/audio/input-cap.html, the input
> capacitor value will be dependent on the series resistor and the
> specified cut off frequency.
>
> Thanks!
>

The first thing to understand is that to a signal, all power rails are
ground. In addition, the inverting input of the opamp is a virtual
ground (if you don't understand that, then google it) so the input
resistance (to an ac signal) is

RinA + (RinB || R3) where || means 'in parallel with'

Cheers

PeteS

PeteS
Guest
Posts: n/a

 11-29-2006, 10:35 PM
PeteS wrote:
> MRW wrote:
>> Sorry for all the questions. I'm just curious.
>>
>> In reading, Texas Instrument's Single Supply Op Amp Circuit Collection
>> guide, I found this attenuator circuit:
>> http://img360.imageshack.us/img360/8...enuatorpj3.jpg .
>>
>> It says that the input resistance should be split into two: RinA and
>> RinB. So I figure that the input resistance would be RinA + RinB.
>>
>> In general, however, how do I go about my analyses to find the input
>> resistance?
>>
>> I get confused easily when looking at the circuits. An example is this
>> page: http://www.geocities.com/ferocious_1...icpreamp2.html ... It
>> says that the two 27k ohm resistors are in parallel. How?
>>
>> The reason why I want to know about the input resistance is because
>> according to http://tangentsoft.net/audio/input-cap.html, the input
>> capacitor value will be dependent on the series resistor and the
>> specified cut off frequency.
>>
>> Thanks!
>>

>
> The first thing to understand is that to a signal, all power rails are
> ground. In addition, the inverting input of the opamp is a virtual
> ground (if you don't understand that, then google it) so the input
> resistance (to an ac signal) is
>
> RinA + (RinB || R3) where || means 'in parallel with'
>
> Cheers
>
> PeteS
>
>

Continuing from that, the cutoff frequency (-3dB) is

1/2 x pi x Rin(eff)x C where Rin(eff) is the effective input resistance
above.

Cheers

PeteS

Andrew Holme
Guest
Posts: n/a

 11-29-2006, 10:38 PM

"MRW" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
> Sorry for all the questions. I'm just curious.
>
> In reading, Texas Instrument's Single Supply Op Amp Circuit Collection
> guide, I found this attenuator circuit:
> http://img360.imageshack.us/img360/8...enuatorpj3.jpg .
>
> It says that the input resistance should be split into two: RinA and
> RinB. So I figure that the input resistance would be RinA + RinB.
>
> In general, however, how do I go about my analyses to find the input
> resistance?
>
> I get confused easily when looking at the circuits. An example is this
> page: http://www.geocities.com/ferocious_1...icpreamp2.html ... It
> says that the two 27k ohm resistors are in parallel. How?
>
> The reason why I want to know about the input resistance is because
> according to http://tangentsoft.net/audio/input-cap.html, the input
> capacitor value will be dependent on the series resistor and the
> specified cut off frequency.
>
> Thanks!
>

You need to consider the AC (signal) and DC (power / bias) conditions
separately.

Power supply rails are ground as far as AC is concerned - so the 27k
resistors are connected in parallel between the input and AC ground.

In the other case, the -ve op-amp input is held at Vcc/2 by the action of
feedback. This is known as a "virtual earth" and is another AC ground. So
the input impedance is RinA + (RinB || R3)

MRW
Guest
Posts: n/a

 11-29-2006, 10:49 PM
Thanks!

I am familiar with simple op amp analysis. The concept of the power
rails being viewed as "ground" by AC signals is new to me. I thought
that author of the site got the 27k ohm resistors to be in parallel
from a Thevenin analysis, but at the same time, I get lost sometimes
with Thevenin.

PeteS
Guest
Posts: n/a

 11-29-2006, 10:59 PM
MRW wrote:
> Thanks!
>
> I am familiar with simple op amp analysis. The concept of the power
> rails being viewed as "ground" by AC signals is new to me. I thought
> that author of the site got the 27k ohm resistors to be in parallel
> from a Thevenin analysis, but at the same time, I get lost sometimes
> with Thevenin.
>

A power rail (voltage) is designed to have a constant voltage. So for
any Delta I (change in current), there should be zero change in voltage,
so we get:

R = V/I = 0/x =0. In addition, power rails are decoupled to ground
through capacitors, which have (simplistically) zero impedance at AC.

Cheers

PeteS

Andrew Holme
Guest
Posts: n/a

 11-29-2006, 11:02 PM

"MRW" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
> Thanks!
>
> I am familiar with simple op amp analysis. The concept of the power
> rails being viewed as "ground" by AC signals is new to me. I thought
> that author of the site got the 27k ohm resistors to be in parallel
> from a Thevenin analysis, but at the same time, I get lost sometimes
> with Thevenin.
>

The 50uF smoothing capacitor across the power supply is an AC short circuit.
Good decoupling makes the power supply an AC ground.

MRW
Guest
Posts: n/a

 11-29-2006, 11:11 PM
PeteS wrote:
> R = V/I = 0/x =0. In addition, power rails are decoupled to ground
> through capacitors, which have (simplistically) zero impedance at AC.

Ahhh.. <light bulb on>... That is right. Sorry, I haven't quite racked
up on experience years to be this insightful.

Thanks!

PeteS
Guest
Posts: n/a

 11-29-2006, 11:11 PM
MRW wrote:
> PeteS wrote:
>> R = V/I = 0/x =0. In addition, power rails are decoupled to ground
>> through capacitors, which have (simplistically) zero impedance at AC.

>
>
> Ahhh.. <light bulb on>... That is right. Sorry, I haven't quite racked
> up on experience years to be this insightful.
>
> Thanks!
>

That's the purpose of s.e.b

Cheers

PeteS

John Popelish
Guest
Posts: n/a

 11-29-2006, 11:20 PM
MRW wrote:
> Sorry for all the questions. I'm just curious.
>
> In reading, Texas Instrument's Single Supply Op Amp Circuit Collection
> guide, I found this attenuator circuit:
> http://img360.imageshack.us/img360/8...enuatorpj3.jpg .
>
> It says that the input resistance should be split into two: RinA and
> RinB. So I figure that the input resistance would be RinA + RinB.
>
> In general, however, how do I go about my analyses to find the input
> resistance?

Think of all fixed voltages (ground, Vcc and Vcc/2) as zero
ohm points as far as signal current is concerned. An
inverting opamp also uses feedback to hold its - input at
the same voltage as is applied to its + input, so that node
can be considered to be a zero ohm to ground node as far as
signals are concerned.

As to the best values for all those resistors, it depends on
what attenuation or gain you want, and whether or not the
output can absorb all the input current through the feedback
resistor, or if you want some of the input current to pass
through R3 to Vcc/2. If you are not concerned with keeping
the input impedance low input impedance to some low value
with R3, R3 is not needed. So the best answer depends on
all the design constraints. lots of combinations will work
under some conditions.

> I get confused easily when looking at the circuits. An example is this
> page: http://www.geocities.com/ferocious_1...icpreamp2.html ... It
> says that the two 27k ohm resistors are in parallel. How?

Each of them loads AC signals with a path to some fixed
voltage. From an AC analysis point of view, all fixed
voltages are ground.

> The reason why I want to know about the input resistance is because
> according to http://tangentsoft.net/audio/input-cap.html, the input
> capacitor value will be dependent on the series resistor and the
> specified cut off frequency.
>
> Thanks!
>

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