Michael Black VE2B V14 16 An wo! h Road Montreal, Quebec Canada H3 Y 2E7

The Amazing Active Attenuator can you find a use for it?

The op amp is the answer.

I'm sure, by now, you have heard of many uses for op amps, But did you know that they can be used as attenuators? For some uses, they are better than attenuators consisting of only resistors. While the principle is very simple, I have not seen much mention of it in the various electronic magazines.

In an inverting op amp amplifier, as shown in Fig, 1, the gain can be varied from the open loop gain down to a gain of one. This gain is determined by the equation Rf/Rjn- where Rf is the resistor from the output to the input and Rjn is the resistor from the driving source to the input of the op amp (Fig, 1), Now, what happens if Rjn becomes greater than Rf? To save you the trouble of running this experiment, the answer is that there is a loss from input to output and the circuit becomes what I call an active attenuator.

There are, unfortunate-

ly, some disadvantages to this attenuator. The first problem is that an active component, along with its associated power supply, is required. Also, frequency response is restricted to the bandwidth of this active component. Next, you must remember that this circuit inverts, so rf you are usmg it for dc, you will get a negative voltage out if you feed in a positive voltage and vice versa.

Now, for the advantages. The output of this active attenuator, as in any op amp amplifier, is a low impedance (independent of resistor values). l~ is means that any loading (within reason) of the output will not change the output voltage. Also, it is great for driving long, unshielded wires without the worry of hum. Also, although I am not yet sure what it is useful for, you can use this circuit as a summing attenuator. This is done by using different input resistors for each input desired. Fig. 2 gives the details,

Here are a few things to watch out for; First, the amplifier configuration must be inverting because a non-inverting op amp amplifier has a minimum gain of one no matter what. Second, the input impedance is equal to Rm. So, when calculating the resistor values, choose Rjn first, so that it is equal to the desired input impedance. Then choose Rf for the desired attenuation. Using this procedure, the resistor equation can be rearranged as Rf = Rjn desired attenuation. Also, make sure that the op amp is still useful at the desired frequency of operation. One thing that I am interested in knowing is what happens to the open-loop frequency response when the circuit is used as an at-

■ 'l j k'. , -V .■: n . i tenuator I know that as the gain of an op amp increases, the frequency


fit fNPUT


fit fNPUT


Fig. 1. Circuit for the active attenuator. VQut = (Rf/Rjn)VmandRjn = Rf X desired attenuation.


Fig. 1. Circuit for the active attenuator. VQut = (Rf/Rjn)VmandRjn = Rf X desired attenuation.

response drops, so with an active attenuator, does the frequency response increase?

Well, i hope that you can put this information to use. I have not given too many details because most of the information required can be found in many magazine articles and books dealing with using op amps, I have found the IC Op-Amp Cookbook by Walter C, Jung (Howard W. Sams & C.o) especially useful. If you come up with some new data or uses for the active attenuator, please let me know about it. if nothing else, you can use this circuit as an answer to those people who go around talking about "passive amplifiers and active attenuators■

c ill

Fig. 2. Circuit for summing attenuator. Attenuation of

Fig. 2. Circuit for summing attenuator. Attenuation of

^in — Rf/Rin7, Bm = Rf/R }n2f and Cin — Rf/Rjn3.

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