# Circuits Question

#### TecHNooB

##### Diamond Member
How come Sallen Key filters use additional circuit elements to enhance/attenuate the gain. Couldn't you use magnitude scaling and get the same result? Or does magnitude scaling not work. I'm currently under the impression that magnitude/frequency scaling works for any linear circuit (given a transfer function).

#### JohnCU

##### Banned
they didnt teach me that in EE

#### TecHNooB

##### Diamond Member
Originally posted by: JohnCU
they didnt teach me that in EE

damnit, i was kind of counting on you

#### vshah

##### Lifer
sadly i can only do ones and zeros

#### Born2bwire

##### Diamond Member
Originally posted by: TecHNooB
How come Sallen Key filters use additional circuit elements to enhance/attenuate the gain. Couldn't you use magnitude scaling and get the same result? Or does magnitude scaling not work. I'm currently under the impression that magnitude/frequency scaling works for any linear circuit (given a transfer function).

Taking a quick look at the wikipedia article, they show the Sallen Key filter having a unit-gain amplifier. The reason for a unity-gain amplifier, at least in the circuit I see on wikipedia, that I can think of is that it isolates the filter from the load. If you place a load on a filter, the load's impedance can affect the filter's characteristics. A unity-gain buffer will show the filter it's infinite input impedance and it will isolate the properties of any load. Thus, the filter is stable across any load that you place on it and you can also assume that the load will be presented a zero impedance from the buffer's output.

It seems TI has an article discussing the filter, why don't you take a read through that? I think though the answer to your question is that the amplifier circuit gives rise to the benefits that I mentioned above.

http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf

#### soydios

##### Platinum Member
magnitude/frequency scaling and the transfer function wasn't explicitly taught to me in my Intro to EE or Intro to Analog Circuits classes. but looking at the circuit diagram on wikipedia:

- you are correct in assuming that the unity-gain buffer is used in this case as a current source to isolate the output and input impedences.
It looks like turning up the gain would be very straightforward: just put a resistor in between Vout and the inverting input to the op-amp. Is that what you're asking?

#### LordMorpheus

##### Diamond Member
Adding a seperate amplifier is not a bad idea, because you can build the seperate amplifier to have a very low output impedence to minimize noise pickup between the amp and the ADC.

Make sure you put the amplifier as close as possible to the filter.

#### PottedMeat

##### Lifer
Originally posted by: Born2bwire
Originally posted by: TecHNooB
How come Sallen Key filters use additional circuit elements to enhance/attenuate the gain. Couldn't you use magnitude scaling and get the same result? Or does magnitude scaling not work. I'm currently under the impression that magnitude/frequency scaling works for any linear circuit (given a transfer function).

Taking a quick look at the wikipedia article, they show the Sallen Key filter having a unit-gain amplifier. The reason for a unity-gain amplifier, at least in the circuit I see on wikipedia, that I can think of is that it isolates the filter from the load. If you place a load on a filter, the load's impedance can affect the filter's characteristics. A unity-gain buffer will show the filter it's infinite input impedance and it will isolate the properties of any load. Thus, the filter is stable across any load that you place on it and you can also assume that the load will be presented a zero impedance from the buffer's output.

It seems TI has an article discussing the filter, why don't you take a read through that? I think though the answer to your question is that the amplifier circuit gives rise to the benefits that I mentioned above.

http://focus.ti.com/lit/an/sloa024b/sloa024b.pdf

Texas Instruments / Analog Devices / National Semiconductor / Maxim have some of the best datasheets and app reports for all sorts of things. If you need some thing to plug into an application chances are they've got a general solution ( not just using their products ) that you can use.