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Exp. 1 Voltage Comparators (I)The function of a voltage comp. is to compare vP against vN, and output either a VOL

or VOH according to

vO = VOL for vP < vN

vO = VOH for vP > vN

a one-bit A/D converter

Define vD = vPvN VTC

At the origin, the VTC indicates an infinite gain actual gain being from 103 to106 V/V.

VOL and VOH may not necessarily be symmetric these two levels be sufficiently

far apart, for example, VOL 0 V and VOH 5 V.

The Response Time

How rapidly a comparator responds as the input state changes from vD < 0 to vD > 0,

and vice versa response time or propagation delay tPD.

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tPD is defined as the time it takes for the output to accomplish 50% of its transition

in response to a predetermined voltage step at the input.

The input step magnitude is typically on the

order of 100 mV its limits are chosen tobarely exceed the input overdrive Vod

Vod is the voltage required to cause the output

to switch states typically 1 mV, 5 mV and

10 mV

In general, tPD decreases with VodtPD can range from a few s to a few ns.

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The Op Amp as a Voltage Comparator

When speed is not critical, an op amp can be an excellent comp. extremely high

gains and low input offsets

A 301 op amp without the internal frequency-compensation capacitor Cc

a threshold detector ( a zero-crossing detector ifVT = 0 V)

No feedback loop vN no longer tracks vP

Slew-rate limit for a 741 op amp, tPD Vsat/SR = (13 V)/(0.5 V/s) = 26 s

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Op amps are intended for negative-feedback operation dynamics are not

optimized for open-loop operation, their output saturation levels are difficult to

interface to digital circuitry.

General-Purpose IC Comparators

LM311

Input stage the pnp emitter followers Q1 and Q2, the differential pair Q3-Q4.

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Output stage amplifiers Q5-Q6 pair and Q7-Q8 pair, base drive of QO

For vP < vN QO = On, for vP > vNQO = Off

The function of Q9 and R5 is to provide overload protection for QO.

When on, QO can draw up to 50 mA. When off, it draws 0.2 nA leakage current.

Both the collector and the emitter terminals of QO are externally accessible to allow

for custom biasing a pullup resistor RC

VCE(sat) 0.1 V

For example, letting VCC(logic) = 5 V and VEE(logic) = 0 V. TTL and CMOS

Letting VCC(logic) = 15 V and VEE(logic) = 15 V stable 15 V outputs

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The 311 can also operate from a single power supply VCC(logic) = VCC is up to 36

V and VEE(logic) = VEE = 0 V.

Biasing with a pulldown

resistor RE QO is an emitter

follower

Response times of the 311

for various input overdrives

Vod = 5 mV

311 is a 200-ns comp.

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DC input errors the input tripping point error EI = VOS + RnIN RpIP

AT 25 oC VOS = 2 mV, IB = (IP + IN)/2 = 100 nA and IOS = IP IN = 6 nA for 311

Nulling

In low-cost single-supply applications LM339 quad comparator

Input stage pnp Darlington pairs Q1-Q2 and Q3-Q4

Current mirror Q5-Q6 forms an active load, a single-ended drive for Q7

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Q7 provides gain and the base drive for the open-collector output transistor QO

suitable for wired-OR operation

When on, QO can sink 16 mA (typ), 6 mA (min). When off, the collector leakage

is 0.1 nA

With a 5.1-k pullup resistance

Typically, VOS = 2 mV IB = 25 nA and IOS = 5 nA, the operating supply range isfrom 2 V to 36 V, and the input voltage range is from 0 V to V CC 1.5 V

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High-Speed Comparators

Response times on the order of 10 ns or less circuit techniques and fabrication

processes high-speed comp. are often equipped with output latch capabilities

vD must be valid at least tS ns (setup time) before the latch-enable command isasserted, and must remain valid for at least tH ns (hold time) thereafter.

Strobe control disables the device by forcing its output stage into a High-Z state

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Experiment 1

1. Using a 311 comp. powered from 15 V supplies:(a) Design a threshold detector such that Vo = 0 V for Vi > 1 V, and Vo = 5 V for

Vi < 1 V.

(b) Repeat (a), but with Vo = 12 V for Vi > 5 V, and Vo = 0 V for Vi < 5 V.