Fundamental Devices:Fundamental Devices:
•• operational amplifieroperational amplifier•• comparator, Schmitt triggercomparator, Schmitt trigger•• linear gateslinear gates
Operational amplifier (op-amp)
Op-amp is a high-gain voltage differential amplifier; typical gain G is in the range 100,000-200,000;
it amplifies the difference in the voltage applied to the pair of inputs
V+, V- positive and negative inputsVout outputVS+ , VS- positive and negative power supply (12-18 Volts)
Note: VS+ same as VDD, VCC, VCC+
VS- same as VSS, VEE, VCC-
• can be used as either an ‘inverting’ (negative feedback) or a ‘non-inverting’ (positive feedback) voltage amplifier
In typical usage op-amp is controlled by negative feedback, as shown:
• no negative feedback, the amplifier is said to be running "open loop"
Because the open-loop gain is typically very large, op-amps are not usually used without negative feedback. Unless the differential input voltage is extremely small, open-loop operation results in op-amp saturation
Inverting op-amp (negative feedback)
Open loop=no feedback
out in open loop
outopen loop
in
V V G
VG
V
= ⋅
=
fout in
i
RV V
R= −
=Rf
=Ri
The ideal op-amp has infinite open-loop gain – this is a model.Real amps only approach this model.
Limitations:
Finite gain;Finite input resistanceNon-zero output resistanceTemperature effects
To a first approximation, the gain of a typical op-amp is inversely proportional to frequency.
Gain is fairly uniform at low frequencies, but tends to fall away at high frequencies
Gain & Frequency
Historical facts: 1952- K2-W first op-amp, G.A. Philbrick lab
1967- LM101 more versatile op-amp version- gain up to 160K and operation range, 'short-circuit' protection, and simplified frequency compensation. In 1968 improved version of LM101 was LM101A, National Semiconductor
1974- RC4558 first multiple op amp device, which uses NPN input transistors, Raytheon Semiconductor's
In dated sequence, the op-amp developed like this: 1963-uA702, 1965-uA709, 1967-LM101/LH101, 1968-uA741, 1974-RC4558/LM324, 1975-CA3130/LF355, and in 1976 the TL084...
Since 1976 the types of op amps have increased almost daily
1963- uA702 first solid-state op-amp by Bob Widlar, Fairchild Semiconductors,supply voltages +12 and -6 volts, burn out when temporarily shorted
1965- uA709 by Bob Widlar, high gain, a larger bandwidth, lower input current, supply voltage of approximately +/- 15 Volt DC
K2-W op-amps from George A.Philbrick’s lab (1952).
Detailed diagram of the common 741 op-amp
Absolute Maximum Parameters (741):
Typical applications of solid-state integrated circuit operational amplifiers:
1 Linear circuit applications1.1 Differential amplifier 1.1.1 Amplified difference1.1.2 Difference amplifier1.2 Inverting amplifier1.3 Non-inverting amplifier1.4 Voltage follower1.5 Summing amplifier1.6 Integrator1.7 Differentiator1.8 Comparator1.9 Instrumentation amplifier1.10 Schmitt trigger1.11 Inductance gyrator1.12 Zero level detector1.13 Negative impedance converter (NIC)
2 Non-linear configurations2.1 Precision rectifier2.2 Peak detector2.3 Logarithmic output2.4 Exponential output
3 Other applicationsaudio and video pre-amplifiers and buffersdifferentiators and integratorsfiltersvoltage regulator and current regulatoranalog-to-digital converterdigital-to-analog converter
The negative feedback connection is the most typical use of an op-amp, but many different configurations are possible, making it one of the most versatile of all electronic building blocks.
Non-inverting op-amp Integrating op-amp Differentiating op-amp
Peak detector
Comparator Schmitt trigger
Comparator
+-
V+
V-
-is a circuit that compares two analog signalsand produces a one bit digital signal
The comparator output satisfies the following rules:-when V+ is larger than V- the output bit is 1.-when V+ is smaller than V- the output bit is 0
Input voltage not exceed the power voltage
-op-amp without feedback
http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML
Comparator- no feedback:
sgn( )out sV V V V+ −= −
1 ,
sgn 0 , 0
1 ,
x
x x
x
− < 0⎛ ⎞⎜ ⎟= =⎜ ⎟⎜ ⎟> 0⎝ ⎠
signum functionwhere
and Vs is the power supply voltage
LM-339 a typical comparator chip.
internal circuitry LM339
Some applications of comparator: constant fraction discriminator, flash ADC
The Schmitt trigger - a comparator with positive feedback
Schmitt trigger has some memory- hysteresis.
Symbol:
1 1
2 2sat sat
R Rfrom V to V
R R − Τ = − Τ =
Vout
Vin
The threshold can be adjusted by controlling
the resistances R1 and R2.
1
2sat
RT V
R=
∆
The choice of ∆ is important: if ∆ is greater than the amplitude of the noise fluctuations stuttering is eliminated.
Τ−Τ
1
0
Threshold T changes
A – input signalB – comparator responseC – trigger response
A
B
C
the effect of using the Schmitt trigger:
Gates -electronic switch controlled by logical signal.
Two basic arrangements:
series
shunt
closedclosedopenopen00
openopenclosedclosed11
gategateswitchswitchsignalsignal
openopenopenopen00
closedclosedclosedclosed11
gategateswitchswitchsignal signal E
E
series
shunt
Open switch exhibit capacitance and closed switch exhibit small resistance.As a result, the high frequency will feed through closed gate.
Consider shunt arrangement:
•Circuits are designed with balanced gates to minimize the pedestal.
•While reducing pedestal short time shifts between balanced gates result in spikes in the output.
Artifact : control signal may influence the output and hence introduce the distortion (submicrosecond scale).
Some applications
1. Multiplexer
In this example, 3-phase time state generator and three linear gates; gates are of series type
2. Event selection
Here, gate signal is generated when event “B” occurs.
3. Op-amp with gate selected feedback impedances
gates are series type;impedance network works as a filter
-+
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