Consulting some questions for op amp application.

1.)  They used a 10M resistor for the input resistor, which sets the effective input impedance for this circuit.  The resistor from IN+ to GND is used for input bias current cancellation.  This was useful for older BJT op amps where the input bias current was well known and always flowed in one direction.  For newer CMOS and JFET amplifiers, or BJT amplifiers with input bias current cancellation you should leave that resistor out.  See the links below for more information:

Input Bias Current Cancelation Resistors—do you really need them?

Do you add a resistor to match the DC resistance at the inputs of your op amp circuits? Check the circuits in figure 1 below. Many of us were instructed to add Rb as “good practice,” making its value equal…

By Bruce Trump over 13 years ago in Archives > The Signal

3 comments

Internal Input Bias Current Cancellation

Other Parts Discussed in Post: OPA209 Last week we reviewed the use of an input bias current cancellation resistor to balance the source resistance at the two inputs of an op amp. The conclusion was that…

By Bruce Trump over 13 years ago in Archives > The Signal

3 comments

2.)  Take a look at the reference designs TIPD101 and TIPD102.  These are more applicable circuits for current sinks. 

3.)  Read through the following articles on current sensing for more information:

1. Current Sensing – Fundamentals
   -EE Times: http://www.eetimes.com/document.asp?doc_id=1279404

2. Current Sensing – Devices
   -EE Times: http://www.eetimes.com/document.asp?doc_id=1279415

3. Current Sensing – Accuracy
   -EE Times: http://www.eetimes.com/document.asp?doc_id=1279451

4. Current Sensing – Layout Guidelines and Troubleshooting
   -EE Times: http://www.eetimes.com/document.asp?doc_id=1279462

Hello,

The high input impedance is achieved with the LM102 input buffer. 

The "fast" feature is achieved with the C2 "feed-forward" capacitor which connects to the comp pins of the LM101A.  See the LM101A datasheet for more information on this feature.  This method was possible on older op amps that included these pins.  You will find that you'll be able to achieve your BW goals with modern op amps without needing to do this.

Hello,

Yes there is.  The bandwidth changes because of the feed-forward capacitance added to the circuit.  This is described in the LM101 product datasheet.

small signal rise time (tr) is equal to roughly 0.35 / (BW).  So faster bandwidths result in faster rise times.  Settling time depends on both the small signal rise time and the circuit phase margin which will determine the overshoot and ringing response.