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

-+