MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Department of Electrical Engineering and Computer Science
6 .121 Bioelectronics Project Laboratory
S . %. Burns
Notes on trigger circuits
Circuits which produce one of two-possible outputs
when the input signals exceed a certain value are variously called triggers, threshold-detectors, level-detectors,
comparators, Schmitt triggers, etc .
Such circuits form
an interface between the continuum of analog signals and
binary world of pulse (or logic) signals .
The trigger
circuit associated with the horizontal sweep system of an
oscilloscope is a common example of a threshold detector .
The static transfer characteristics of a simple threshold
detector are given in figure 1 .
figure 1
The transfer character-
figure 2
istic illustrated -in figure 1 can be . realized in many ways .
For example, the operational amplifier circuit shown in
figure 2 has such a transfer characteristic .
Static Parameters
The salient parameters describing a threshold detector
are the threshold level, Vth, the high logic level output,
VH, the low logic level output, VL, and the range of input
signals which produce an ambiguous output
to the slope of the transfer curve
fier) .
or
(this is related
the gain of the ampli-
This range is sometimes called the linear range
since the incremental output voltage is linearly related
to the incremental input voltage .
Of course the object
is to minimize this ambiguous range i .e . to maximize the
amplifier gain, A .
The high and low output levels, VH and .
VL respectively, are specified by the type of pulse (logic)
circuitry which is to be actuated by the threshold detector .
For NPN transistor logic such as the 900 series
(Resistor-Tran-
sistor-Logic) RTL of integrated circuit digital logic,
0 .3 volts is considered logical low,
less than
and greater than
1 .2 v is considered logical high . For the 930 DTL (DiodeTransitor-Logic)
or the 7400 TTL (Transitor-Transitor-Logic)
series of intearated circuit digital logic elements, logical
low is less than 0 .4 volts and logical high is greater than
2 .6 volts .
The threshold voltage, Vth,
is a variable which is
usually dependent on the particular measurement we are making .
Differential Amplifiers as Comparators
When using a differential amplifier as a comparator
there are two limitations on Vth .
The common mode voltage
range determines the upper and lower limits of
Vin + Vth , and the maximum differential input limits
IVth -
2
In addition, the common-mode rejection ratio is often of
considerable importance .
The maximum output produced by
Vinl'
a common -mode signal equal to the threshold voltage,
Vth , must not cause an erroneous output .
Typically the
CMRR must be at least 20 db bett.er than the gain of the
amplifier .
Dynamic Parameters
Also of importance in threshold detector is the speed with
which they respond to a change in input signal .
Figure 3
illustrates the dynamic parameters of a comparator .
No-
tice that there is a delay between the change in input and
the change in output voltage and that these delays are
usually not equal for rising and falling inputs .
fvmi
It should
be pointed out that threshold
circuits or comparators are dealing
with large signals . . . signals which
are large enough to saturate the
amplifier .
figure 3
Thus delays and switching
times obtained when using an opera-
tional amplifier which claims to have a 25 megacycle response
are not of the order of
(2 .5 x 10 7 ) -l sec .
Charge storage
times are usually quite significant in an amplifier designed
for linear operation and delays of the order of microseconds
should be expected when the amplifier operates outside its
linear range .
Further, the slew-rate (the rate of change of
output voltage in an operational amplifier) is often limited
to the order of a few volts per microsecond so we expect
rather slow transistions .
Hysteretic Comparators
Hysteretic comparators exhibit hysteresis
(memory) .
Their present state depends on the input voltage and on the
previous state of the device .
The transfer characteristic
and circuit implementation of such a device are illustrated
in figures 4 and 5 .
Hysteretic comparators have both advantages and disadvantages
when compared with the memoryless comparator .
The advantages
of the hysteretic comparator include its sharp threshold, high
temporal resolution and its noise immunity and freedome, from
oscillation .
Some disadvantages are its multiply-defined
input-output relationship and the need to be concerned with
two threshold values .
The sharp threshold .of the hysteretic comparator is caused
by positive feedback .
The effective closed-loop gain is increased
and thus the transition region is smaller .
Improved noise im
munity comes about because the hysteretic comparator is answering a different (and perhaps more relevant question)
does the memoryless comparator .
than
The hysteretic comparator
tells us when the input voltage first exceeds the upper threshold
level,
given that it was less than the lower threshold
level whereas the memoryless comparator tells us only
when the input voltage exceeds the threshold level .
If there is an amount of noise superposed on the signal
which exceeds the threshold range of the memoryless comparator then the output will change back and forth with
the noise signal .
In the hysteretic comparator this
noise voltage must exceed the difference between the two
thresholds to have any effect on the output .
One could
argue that when we detect the output of the comparator
we can respond the first transistion and ignore subsequent
output which we know is due to noise .
This argument
is true but sometimes the high-frequency signal caused by
the oscillatory comparator can produce considerable trouble
in other parts of a system because of coupled noise, etc .
Good engineering practice minimizes the unpredictable .
An example of the outputs of the two comparator systems
on the same noisy signal is illustrated in figure 6
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c,
figure 6
6 An integrated circuit comparator is available .
This unit is packaged in an
in
eight-pin mini-dip (Dual In-line Package) and is described
facturer's specification included as an appendix.
detail in the manu-
(See the Lr3ll data sheet.)
Figure 7
High-speed compaarators, such as the 17,311, are prone to oscillation and instability unless precautions are taken with power supplies .
The positive(and necg-
ative)supplies (pins 8 and(4)respectively) are decoupled from other circuit
elements by a resistor (27 ohm) capacitor (0 .1 mF) decoupling network .
The re-
sistors min ui ze interference on the power supply from affecting the comparator,
and the capacitor provides a local source of energy when the unit is switching .
Even-with these precautions you may find that the comparator oscillates when
biased into its linear range .
This might be expected, for the unit was designed
as a co=arator and should be in the linear region only fleetingly .
Hysteresis
will decrease this propensity by reducing the linear range (increasing the gain
with positive feedback) .
Beware!
You cannot see the high-frequency activity
of the 311 with low-frequency oscilloscopes .
scope if it behaves strangely .
Check it out with a high frequency
The output circuit of the 0311 is a transistor switch shown in Figure 8a .
Functional equivalent is the circuit shown in Figure 8b.
ACTUAL
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Fig . 8b
Fig . 8a
To generate a voltage pulse, this_ switch must be appropriately connected .
For example, we can connect the switch between the positive supply and ground,
as illustrated in Figure 7 .
Other connections are possible and are illustrated
in the applications notes accompanying the manufacturers data sheets .
The circuit shown in Figure 9 might be used as a trigger generator for
electrocardiograms .
Note that it generates a negative (normally high) pulse
whose duration is affected by the detailed nature of the signal .
Normal prac
tise would use an edge of this pulse as the basis of time measurements .
Without an awesome amount of thought, you can see a number of problems
with this EKG detector .
What if the EKG pulse was almost entirely negative?
How do you adjust the threshold?
it be tailored to the patient?
Can one adjustment serve all patients or must