Wideband Gain Block Amplifier Design Techniques

−0.358050PHT( f)100150150.9992000 5. 10 9 1 . 10 10 1.5 . 10 10 2 . 10 10110 x7f2x10 10▲ Figure 2(a). Darlington amplifier loop gain plot.▲ Figure 2(b). Darlington amplifier loop gain phase plot.1513.32300251.15810250AcldB( f)5Rin( f)Ro ut ( f)2001500100−1.35850 5 . 10 9 1 . 10 10 1.5 . 10 10 2 . 10 10110 x7f210 x1050 50110 x7 f 210 100 5 10 9 1 10 10 1.5 . 10 10 2 . 10 10▲ Figure 3. Darlington amplifier closed loop gain dB magnitudeplot.▲ Figure 4. Plot of the Darlington amplifier input/outputimpedance vs. frequency.ance and approximately equal parasitic shunt capacitance.Therefore, the loop gain expression has a dominantdouble pole, which causes excessive phase shift andresults in low amplifier phase margin. The zeros at f z1and f z2 tend to neutralize the poles at f T1 and f T2 bydecreasing loop gain phase shift. Stability against oscillationsis secured because the low mid-band loop gainvalue will reach its unity gain frequency before loop gainphase shift reaches 180 degrees, as shown in Figures2(a) and 2(b). This results in a stable design thatexhibits the gain peaking frequency response as shownin Figure 3.The amplifier-closed loop gain frequency responseexhibits a very flat response with 2 dB peaking and a 3dB bandwidth of 9.8 GHz. Equation (2) correctly predictsthe gain roll off seen in Figure 3 and shows thatthis decrease in closed loop gain approaches zero as Tapproaches zero. Adding series resistance to the base ofQ 1 further reduces loop gain phase shift. The value ofthis series resistance is found through circuit simulations.This simple solution improves phase margin andreduces frequency peaking by effectively adding a lowpass filter to the amplifier’s frequency response.The maximum stable bandwidth of the amplifier islimited by the unity current gain frequencies of devicesQ 1 and Q 2 . These device-induced poles in Equation (9)are essentially fixed depending on bias conditions.Attempts to improve bandwidth by decreasinginput/output time constants will produce amplifierinstabilities when the dominant double pole frequencyapproaches f T1 and f T2 . Bandwidth can be slightlyimproved with careful choice of package type and PCBlayout, but care must be taken in order to maintainamplifier stability. RF Micro Devices’ GaAs HBT technologypossesses an f T approaching 30 GHz, which issufficient for this design.The frequency response of the input impedance isfound by substituting Equation (9) into Equation (3)and Equation (4) for the output impedance. Theinput/output impedance is set to 50 ohms by the loopgain (very precisely for low frequencies), but increaseswith decreasing loop gain, as shown in Figure 4. This104 · APPLIED MICROWAVE & WIRELESS