LT1227 WWISPLI IF ED SCHE ATIC 7 V + 14k NULL 1 NULL 5 CURRENT SOURCE BIAS 8 S/D +IN 3 2 –IN 6 VOUT 4 V– 1227 SS OUUWUAPPLICATIS I FOR ATIO The LT1227 is a very fast current feedback amplifier. 5dB of peaking. The curves stop where the response has Because it is a current feedback amplifier, the bandwidth more than 5dB of peaking. is maintained over a wide range of voltage gains. The At a gain of two, on ±15V supplies with a 1k feedback amplifier is designed to drive low impedance loads such as resistor, the bandwidth into a light load is over 140MHz, cables with excellent linearity at high frequencies. but into a heavy load the bandwidth reduces to 120MHz. The loading has this effect because there is a mild reso- Feedback Resistor Selection nance in the output stage that enhances the bandwidth at The small-signal bandwidth of the LT1227 is set by the light loads but has its Q reduced by the heavy load. This external feedback resistors and the internal junction ca- enhancement is only useful at low gain settlings; at a gain pacitors. As a result, the bandwidth is a function of the of ten it does not boost the bandwidth. At unity gain, the supply voltage, the value of the feedback resistor, the enhancement is so effective the value of the feedback closed-loop gain and load resistor. The characteristic resistor has very little effect. At very high closed-loop curves of Bandwidth vs Supply Voltage show the effect of gains, the bandwidth is limited by the gain bandwidth a heavy load (100Ω) and a light load (1k). These curves product of about 1GHz. The curves show that the band- use a solid line when the response has less than 0.5dB of width at a closed-loop gain of 100 is 12MHz, only one tenth peaking and a dashed line when the response has 0.5dB to what it is at a gain of two. 7
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