Page 94 - The Art and Science of Analog Circuit Design

Steve Roach

The overdrive recovery performance of a two-path amplifier can be
abysmal. There are two ways in which overdrive problems occur. If a
signal is large enough to turn on one of the protection diodes, Cl charges
very quickly through the low impedance of the diode (Figure 7-13). As if
it were not bad enough that the input impedance in overdrive looks like
270pF, recovery occurs with a time constant of 270pF -4.7MQ, or 1.3ms!
Feedback around the op amp actually accelerates recovery somewhat but
recovery still takes eons compared to the 400ps rise time! Another over-
drive mechanism is saturation of the source follower. When saturation
occurs, the op amp integrates the error it sees between the input and
source follower output, charging its 6.8nF feedback capacitor. Recovery
occurs over milliseconds. The seriousness of these overdrive recovery
problems is mitigated by the fact that with careful design it can take ap-
proximately ±2V to saturate the MOSFET and ±5V to activate the pro-
tection diodes. Thus, to overdrive the system, it takes a signal about ten
times the full-scale input range of the pre-amp.
I apologize for turning a simple, elegant, single transistor source fol-
lower into the "bootstrapped, two-path impedance converter." But as I
stated at the beginning, it is the combination of requirements that drives
us to such extremes. It is very hard to meet all the requirements at once
with a simple circuit. In the next section, I will extend the two-path tech-
nique to the attenuator to great advantage. Perhaps there the two-path
method will fully justify its complexity.

I have expended a large number of words and pictures on the impedance
converter, so I will more briefly describe the attenuator. I will confine
myself to an introduction to the design and performance issues and then
illustrate some interesting alternatives for constructing attenuators. The
purpose of the attenuator is to reduce the dynamic range requirements
placed on the impedance converter and pre-amp. The attenuator must
handle stresses as high as ±400V, as well as electrostatic discharge. The
attenuator maintains a 1MO input resistance on all ranges and attains
microwave bandwidths with excellent flatness. No small-signal micro-
wave semiconductors can survive the high input voltages, so high-
frequency oscilloscope attenuators are built with all passive components
and electromechanical relays for switches.
Figure 7-15 is a simplified schematic of a 1MQ attenuator. It uses two
stages of the well-known "compensated voltage divider" circuit. One
stage divides by five and the other by 25, so that division ratios of 1, 5,
25, and 125 are possible. There are two key requirements for the attenua-
tor. First, as shown in Figure 7-3, we must maintain RjQ = R 2C 2 in the
™5 stage to achieve a flat frequency response. A similar requirement
holds for the -f 25 stage. Second, the input resistance and capacitance at
each stage must match those of the impedance converter and remain very

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