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8 Small-signal high-speed amplifiers
RL
M2 Cµ1
+ vs −
+ vo − Cdb + Cµ2 + CL
M1
Figure 8.5 A resistively loaded cascode amplifier does not suffer from the Miller effect and is much more stable due to reduced feedback.
The gain-bandwidth product of the amplifier is thus bounded by Av × ω−3 d B =
gm gm ≈ Cµ,2 + Cdb,2 + C L CL
(8.19)
For stability reasons, we typically operate this amplifier at a frequency sufficiently lower than ωT and thus C L is made large enough to ensure this condition. In theory, therefore, this amplifier has a gain-bandwidth product approaching a significant fraction of the ωT of the device. There are a few problems with this amplifier. First, if we are greedy for gain, we have to pay with headroom since a larger load resistor R L consumes larger DC headroom. This may lead to unreasonably high supply voltages. In most applications, we do not have control over the supply due to the intrinsic breakdown in a transistor. Higher f T devices also have lower breakdown voltages, leading to a natural limit to the gain of the amplifier. For example, a 130 nm CMOS technology may limit the supply to 1.3 V. In analog circuits the voltage headroom is usually solved by employing active loads. Thus the upper limit of gain is set by the maximum current drive or power dissipation. A cascode amplifier operating with 10mA of bias current, therefore, cannot have a load resistor larger than about 130 . A transistor in saturation can obtain an output impedance ro much larger than Vds /Ids . Gain boosting a cascode load, for instance, can greatly enhance the output impedance. But an active load has several drawbacks. First it further limits the output swing of the amplifier since operation into the triode region must be avoided for both the load and the cascode transconductance device. In many applications a large swing is necessary (say in a LO buffer or PA driver amplifier). Furthermore, the non-linearity of the load degrades the linearity of the amplifier, leading to excess distortion.