LT1223 UUWUAPPLICATIO S I FOR ATIO have about 2.5mV thermal tail on a 10V step. Unfortu- Accurate Bandwidth Limiting the LT1223 nately, reducing the input signal and increasing the gain always results in a thermal tail of about the same amount It is very common to limit the bandwidth of an op amp by for a given output step. For this reason we show separate putting a small capacitor in parallel with RF. DO NOT PUT graphs of 10mV and 1mV non-inverting amplifier settling A SMALL CAPACITOR FROM THE INVERTING INPUT OF times. Just as the bandwidth of the LT1223 is fairly A CURRENT FEEDBACK AMPLIFIER TO ANYWHERE ELSE, constant for various closed-loop gains, the settling time ESPECIALLY NOT TO THE OUTPUT. The capacitor on the remains constant as well. inverting input will cause peaking or oscillations. If you need to limit the bandwidth of a current feedback amplifier, Adjustable Gain Amplifier use a resistor and capacitor at the noninverting input (R1 and C1). This technique will also cancel (to a degree) the To make a variable gain amplifier with the LT1223, vary the peaking caused by stray capacitance at the inverting input. value of RG. The implementation of RG can be a pot, a light Unfortunately, this will not limit the output noise the way controlled resistor, a FET, or any other low capacitance it does for the op amp. variable resistor. The value of RF should not be varied to change the gain. If RF is changed, then the bandwidth will R1 be reduced at maximum gain and the circuit will oscillate VIN + when RF is very small. C1 LT1223 OUT V – VIN + R1 = 300Ω RF LT1223 C1 = 100pF OUT V BW = 5MHz – RG RF LT1223 • TA05 RG LT1223 • TA03 Current Feedback Amplifier IntegratorAdjustable Bandwidth Amplifier Since we remember that the inverting input wants to see Because the resistance at the inverting input determines a resistor, we can add one to the standard integrator the bandwidth of the LT1223, an adjustable bandwidth circuit. This generates a new summing node where we can circuit can be made easily. The gain is set as before with apply capacitive feedback. The LT1223 integrator has RF and RG; the bandwidth is maximum when the variable excellent large signal capability and accurate phase shift at resistor is at a minimum. high frequencies. V + IN + LT1223 LT1223 OUT V OUT V OUT V 1 – = – IN V sC1R1 R 5k F 1k C1 R R1 G RF IN V LT1223 • TA06 LT1223 • TA04 1223fb 10
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