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LT1230 Datasheet(PDF) 8 Page - Linear Technology |
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LT1230 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LT1229/LT1230 8 limited by the gain bandwidth product of about 1GHz. The curves show that the bandwidth at a closed-loop gain of 100 is 10MHz, only one tenth what it is at a gain of two. Capacitance on the Inverting Input Current feedback amplifiers want resistive feedback from the output to the inverting input for stable operation. Take care to minimize the stray capacitance between the output and the inverting input. Capacitance on the inverting input to ground will cause peaking in the frequency response (and overshoot in the transient response), but it does not degrade the stability of the amplifier. The amount of capacitance that is necessary to cause peaking is a func- tion of the closed-loop gain taken. The higher the gain, the more capacitance is required to cause peaking. We can add capacitance from the inverting input to ground to increase the bandwidth in high gain applications. For example, in this gain of 100 application, the bandwidth can be increased from 10MHz to 17MHz by adding a 2200pF capacitor. LT1229 • TA05 – + CG RG 5.1 Ω RF 510 Ω VOUT 1/2 LT1229 VIN Boosting Bandwidth of High Gain Amplifier with Capacitance on Inverting Input FREQUENCY (MHz) 1 19 22 25 28 31 46 49 10 100 LT1229 • TA06 34 37 40 43 CG = 4700pF CG = 2200pF CG = 0 S APPLICATI I FOR ATIO The LT1229/LT1230 are very fast dual and quad current feedback amplifiers. Because they are current feedback amplifiers, they maintain their wide bandwidth over a wide range of voltage gains. These amplifiers are designed to drive low impedance loads such as cables with excellent linearity at high frequencies. Feedback Resistor Selection The small-signal bandwidth of the LT1229/LT1230 is set by the external feedback resistors and the internal junction capacitors. As a result, the bandwidth is a function of the supply voltage, the value of the feedback resistor, the closed-loop gain and load resistor. The characteristic curves of Bandwidth versus Supply Voltage are done with a heavy load (100 Ω)andalightload(1k)toshowtheeffect of loading. These graphs also show the family of curves that result from various values of the feedback resistor. These curves use a solid line when the response has less than 0.5dB of peaking and a dashed line when the re- sponse has 0.5dB to 5dB of peaking. The curves stop where the response has more than 5dB of peaking. Small-Signal Rise Time with RF = RG = 750Ω, VS = ±15V, and RL = 100Ω LT1229 • TA04 At a gain of two, on ±15V supplies with a 750Ω feedback resistor, the bandwidth into a light load is over 160MHz without peaking, but into a heavy load the bandwidth reduces to 100MHz. The loading has so much effect because there is a mild resonance in the output stage that enhances the bandwidth at light loads but has its Q reduced by the heavy load. This enhancement is only useful at low gain settings; at a gain of ten it does not boost the bandwidth. At unity gain, the enhancement is so effective the value of the feedback resistor has very little effect. At very high closed-loop gains, the bandwidth is |
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