CHAPTER 6: BROADBAND AMPLIFIERS Broadband is more than one octave -3dB pass band Second order harmonics are in the pass band PASSIVE BROAD BANDING CS amplifier and Miller effect Miller effect causes a dominant pole at the input when source impedance and vgain are high, But when CL*RL >> 1/omegaM, this will be the dominant pole Remedies: Add an inductor in series with the collector/drain resistor Inductor impedance rises with frequency new BW limit is LC resonant f Shunt peaked amplifier Load impedance: choose the inductance Check 3dB BW difference with and without L=> omega2/omega1 Select a criterion: Max BW, ZL = R at omega1 amplitude flatness, flat group delay Series peaked amplifier Separating the load capacitance with series inductance No LF zero Combination (plus extra L) Shunt and double series peaking Shunt peaked L is biggest, has most delay So a change in CGD can load first, than later CL so faster rise time A trade-off between BW and delay Three coils can be replaced by one T-coil Pole-zero cancellation ONLY for low order networks Example of the probe We use a tuneable capacitor So the TF is a pole zero doublet Other approach Introduce a zero in a CS amplifier at the source LF: gain is reduced; HF gain rises due to lower impedance of C Chose R.C = RL.CL dominant (if the case) is cancelled Method of open-circuit time constants How to search the dominant pole? => Open-circuit time-constants OCtau Underestimates the true bandwidth, accurate for dominant pole systems OCtau does not take imaginary parts in to account Beware: some C are not BW limiting (coupling C => short them) Inductance: short L, add the L/R tau’s afterwards (but L generate complex pole) FEEDBACK TECHNIQUES The basics Negative feedback lowers closed-loop gain Transc, current => ideal current source others voltage source Voltage Transconductance Impact of negative feedback on the in/output impedance transimpedance current Impact of negative feedback on linearity Higher feedback gain => linearization Impact of negative feedback on noise Noisy amplifier will introduce noise in the network; feedback can’t improve that (only worsen it) vEN inputs shorted, outputs set equal Voltage feedback opamp: compensation Bandwidth is extended by T (A*B) A: 1st order Compensation: make opamp with dominant pole BW limitation linearity problems poorer slew rate (cannot be improved by feedback) Current feedback opamp: speed (is better than voltage) + input => high ZIN; - input => LOW ZIN (voltage buffer!) Bandwidth and closed loop gain are independent Little slew rate limitation Series vs. Shunt resp. feedback on 1 transistor Shunt-series feedback Effective transconductance Design procedure: Av en R Rf => input impedance Re => gain Consider for HF a perfect BJT (rpi & r0 = 0) Multi stage feedback Dual transistor broadband amplifier Problem: every stage causes phase shift Stability issue Worst case scenario for stability: B equals 1 Bode stability criterion: starting at phase(A) = 0° phase crossover frequency (omegaPC) when phase(A) = -180° gain crossover frequency (omegaGC) when log(mod(A)) = 0 dB At omegaPC, the loop gain A(omegaPC) must be less than 0dB 1st order: always stable when B <= 1 Caveat Not always able to determine A, nor B Return ratio analysis We look for the return ratio of the error-amplifier Caveat: an open-loop interrupts any back-propagating signals (unilaterality is assumed) So look for the best unilateral device CE/S or even better a cascade output ACTIVE BROAD BANDING Passive is expensive (large area) and, not very good/accurate Active components are cheaper, can match each other better Miller effect (The effective input capacitance of an inverting amplifier rises due to negative feedback caused by capacitance between the input and output) The input impedance forms together with the capacitance a LPF Miller pole: Miller effect: non ideal amp Use small scale approximation. Use unilateral approximation (UA) Neglect: I that leaves collector through CCB, But don’t neglect the I that leaves the base Reduce Miller effect 1: Cascode amplifier We use a CS and a CG ZIN of M2 is low => reduction in voltage gain of the CS Result: lower voltage gain and Miller multiplication Use Source substitution: replace the source in series by a new source in shunt + resistor Conclusion: BW higher, input pole is no longer dominant but more complex and reduced output level swing and more dissipation Reduce Miller effect 2: Source coupled amplifier Prevents the miller effect (is halved) by eliminating an voltage inversion Reduce Miller effect 3: Neutralization Positive feedback: (STABILITY still OK??) = compensation of the miller effect Inject an inverted C current into the input node to cancel the Miller C current Voltage at the top is in anti phase with ID CN (= CGD) feeds this negative current So the INET from the gate in CN+CGD is zero Needs to be differential, care for positive feedback FT doublers: clever circuits with increased speed Lower input C, but gM must stay equal Differential pair => 2 identical C in series But three devices (including current source) Not possible? Use Battjes doubler Looks like Darlington but different bias Same technique as differential pair, Bootstrapping Form of positive feedback Create a node with very high impedance Two examples (left) High impedance at M2 drain so Cgs becomes useless (right) CGD is also bootstrapped by buffer A1