Electronotes/AN-23 - The CA3080 as a voltage-controlled resistor: Difference between revisions
Electronotes/AN-23 - The CA3080 as a voltage-controlled resistor (view source)
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[[File:AN23 fig 1a.jpg|thumb|right|250px|Fig. 1a.jpg]][[File:AN23 fig 1b.jpg|thumb|right|250px|Fig. 1b.jpg]][[File:AN23 fig 1c.jpg|thumb|right|250px|Fig. 1c.jpg]][[File:AN23 fig 1d.jpg|thumb|right|250px|Fig. 1d.jpg]][[File:AN23 fig 1e.jpg|thumb|right|250px|Fig. 1e.jpg]][[File:AN23 fig 2a.jpg|thumb|right|250px|Fig. 2a.jpg]][[File:AN23 fig 2b.jpg|thumb|right|250px|Fig. 2b.jpg]]In [[Electronotes/AN-22|AN-22]], we looked at the [[CA3080]]
<math>\frac{V_{diff}}{I_{out}} = \frac{1}{(19.2 \cdot I_{c
We can thus look at the circuit of Fig.
<math>I_{out} = 19.2 \cdot I_{c} \cdot V_{diff} = \frac{I_c \cdot V_{in}}{23.7} = \frac{V_{in}}{R_{eq}}</math>
where <math>R_{eq} = \frac{23.7}{I_{c}}</math>, and where we have made use of the values shown for the [[voltage divider]] [[attenuator]]. What kind of a VCR is this? Well, we can see that the ground point receives current as though it were being supplied from a voltage <math>V_{in}</math> through a resistor <math>R_{eq}</math> is shown in Fig.
Next we would like to look to see if we can make a VCR that looks like a resistive load. That is, we want to have a VCR that actually draws different currents depending on the voltage across it - which after all is what a real resistor does. This can be implemented as shown in Fig. 2a. Ignore for the moment the upper op-amp in Fig. 2a, which is just a [[operational amplifier buffer|voltage follower]] to drive the attenuator on the input of the CA3080. The output of the CA3080 is thus
<math>I_{out} = I_{c} \cdot \frac{V_{in}}{23.7} = \frac{V_{in}}{R_{eq}}</math>
which is the same thing we had above. But here the current is drawn from the input (<math>I_{in} = I_{out}</math>) so the VCR looks like a resistor to ground, as seen in Fig. 2b. We can thus use this sort of VCR to implement the [[high pass filter|high-pass filter]] structure shown in Fig. 2c. The implementation is shown in Fig. 2d, and the 3dB frequency is <math>\frac{1}{2 \pi R_{eq}C}</math>.
<div><ul>
<li style="display: inline-block;">[[File:AN23 fig 2c.jpg|thumb|right|250px|Fig. 2c.jpg]]</li>
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<li style="display: inline-block;">[[File:AN23 fig 3b.jpg|thumb|right|250px|Fig. 3b.jpg]]</li>
</ul></div>
Since we have implemented the simple R-C high-pass filter (with an output buffer), it is of interest to ask if the corresponding R-C [[low pass filter|low-pass filter]] can be realized. It might at first seem that the VCR of Fig.
<math>I_{out} = 19.2 \cdot I_{c} \cdot V_{diff} = 19.2 \cdot I_{c} (V_{+} - V_{-}) = \frac{I_{c}(V_{in} - V_{out})}{23.7} = \frac{(V_{in} - V_{out})}{R_{eq}}</math>
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<math>I_{R} = \frac{(V_{in}-x)}{R} = \frac{(V_{in}-V_{out})}{R}</math>
Thus we have implemented three types of VCR's. The first (Fig.
A more general form of a floating resistor is shown in Fig. 4, and is a circuit first suggested by G. Wilcox.
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<math>I = (V_{2}-V_{1})(\frac{1}{200k}+\frac{I_{c}}{47.3})</math>
Thus, as the current <math>I_{c}</math> is increased, the effective resistance goes down, starting from a value of 200k (or whatever attenuator is used). For a completely linear system, the circuit of Fig. 5 can be used to get rid of the
== References ==
*
[[Category:Electronotes Application Notes]]
[[Category:CA3080]]
[[Category:OTA]]
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