Remember the equation for this circuit is:

V_out = V_in * (1 + R_f/R_i)

(Please see my archives for an intuitive explanation of why this is the equation for this circuit.)

Then insert a capacitor in series with R_i like this.

Figure 2 - Standard Noninverting Op-Amp with Capacitor Added

Now, let's rewrite the equation for the amplifier first.

V_out = V_in * (1 + Z_f/Z_i)

Consider the elements in the feedback loops to be impedances in place of resistances. Now it should be obvious that:

Z_i = R_i + the impedance of C_i

So to put it simply, the impedance of Z_i will approach R_i as frequency increases, and infinity as the frequency approaches 0 or a DC level. Now insert that into the original equation. What happens?

V_out = V_in * (1 + Z_f/(R_i + impedance of C_i)

As the frequency approaches a DC level, the gain of the circuit approaches 1. As the frequency increases, the gain approaches 1+Z_f/Z_i. You now have an amplifier that only amplifies AC signals. Any DC bias that was on the signal to begin with is simply passed on to the output. This can be a very handy characteristic. There are many instances where the AC signal you are interested in is riding on a DC bias. If you amplify it a lot, you will run into a problem where the op-amp will "rail" and cease to operate as desired. You can work out a special ground and a bipolar supply to eliminate it, or you can use the above circuit. I personally prefer the latter wherever possible.

A Voltage-Controlled, Grounded Constant-Current Source
A few articles back, in response to a reader's question, I tried to find this circuit on the Net so I could send the reader the link. Problem was, I couldn't find it. So here it is for those of you that need a constant-current source.

Figure 3 - Voltage-Controlled, Grounded Constant-Current Source

So let me explain the operation of this circuit a little. Remember the negative feedback rule? If there is negative feedback, the op-amp will try to make V+ = V- (if you think there is any significant current flowing in or out of these inputs, you better go read the archives). Well in this case there is negative feedback. Follow my logic for a moment. (Yes, I did use the term logic in reference to an analog circuit. Logic implies rules and even analog circuits must follow rules. However, I am sure there are engineers that would dispute this assertion. J)

We begin at the output of the op-amp. As the voltage at the output of the op-amp decreases, current through the transistor will increase. This means the voltage at V- will decrease as well, and the current through R will go up because it is connected to the transistor. But, if V- falls below V+, then the error signal in the op-amp is positive, and that will make the output of the op-amp increase voltage. This process will stabilize when V- = V+. Tada! The magic of negative feedback works again. That's great you say, but I thought we were talking about a constant-current source. We are, and that is the beauty of this design. If V- = V+, that means that the voltage across R is always equal to Vcc - V_in. You simply apply Ohm's law to discover that

Current = (Vcc - V_in)/R

Since the current through R is the same as the current through "Load," you have a constant-current source. Even better, you can adjust the current by simply changing V_in. Another great thing is that this circuit works with a load that is connected to ground. A good example as to where this type of circuit could be used is to hook up a micro-controller to switch a standard 4-20 mA current loop interface.

A couple of words of warning, however. A constant-current source will not control current unless a load is connected. It is also possible to connect a load that will force the source out of regulation. So make sure you know the maximum range required as well as the total available current. (All this depends on the implementation, the value of Vcc, the value chosen for R and R_b, etc.) Note also that current increases as V_in decreases. (Look at the above equation again if you didn't notice that it is referenced to Vcc).