Datasheet LT1229, LT1230 (Analog Devices) - 8

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