Transcript Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 15
Special
ICs
Objectives
Describe and Analyze:
• Common Mode vs. Differential
• Instrumentation Amps
• Optoisolators
• VCOs & PLLs
• Other Special ICs
Introduction
• This chapter examines some important op-amp
related topics such as common-mode rejection.
• It also examines some non op-amp linear circuits
such as Voltage Controlled Oscillators (VCOs) and
Phase-Locked Loops (PLLs)
Single-Ended vs. Differential
A signal applied between an input and ground is called
a single-ended signal.
A signal applied from one input to the other input is
called a differential signal.
Differential Amplifier
Resistances must be symmetric for a diff-amp.
Common-Mode Signals
• Ground-referenced signals applied simultaneously
to both inputs of a diff-amp are common-mode
signals.
• Electrical noise and interference often appear as
common-mode signals.
• Signals from transducers are usually differential.
• To extract small differential signals out of a “soup” of
common-mode noise, a diff-amp requires a high
common-mode rejection ratio (CMRR).
Definition of CMRR
• The common-mode rejection ratio (CMRR) of a diffamp is defined as:
CMRR = 20 Log(AV(diff) / AV(cm))
• where AV(diff) is the voltage gain for differential
signals and AV(cm) is the gain for common-mode
signals.
• A perfect diff-amp would have AV(cm) equal to zero,
so it would have infinite CMRR.
• Real diff-amps have CMRRs in the range of 90 dB
to 110 dB or better.
Example Calculation 1
• Find the CMRR required so that differential signals
have a gain of 100 and common-mode signals have
a gain of 0.001 (an attenuation)
CMRR = 20 Log(AV(diff) / AV(cm))
= 20 Log(100 / 0.001)
= 20 Log(100,000)
= 20 Log(105)
= 20  5
= 100 dB
CMRR is less if the external resistors are not matched.
Example Calculation 2
• A diff-amp has a gain of 10 and a CMRR of 80 dB.
The input is a differential signal of 1 mV on top of
1 Volt of common-noise. How much signal voltage,
and how much noise voltage, will be at the output of
the diff-amp?
CMRR = 20 Log(AV(diff) / AV(cm))
So AV(cm) = AV(diff) / Log-1(CMRR/20)
= 10 / Log-1(80/20) = 10 / 104 = 10-3 = 0.001
So at the output there will be 10 mV of signal
and 1 mV of noise
Instrumentation Amps
Except for Ri, all the above can be on one chip.
Instrumentation Amps
Advantages of instrumentation amplifiers are:
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Gain set by one resistor
High CMRR
High Zin on both input pins
Work well with most transducers
Transconductance Amps
• Operational transconductance amplifiers (OTAs) look like other
op-amps, but the output is a current instead of a voltage.
• Gain is a transconductance (mutual-conductance)
gm = iout / Vin
• The value of gm is proportional to a DC bias current:
gm = K  IB
• OTAs have relatively wide bandwidth.
• OTAs have high output impedance (Zout).
• The gain control by a current allows one signal to multiply
another.
Optoisolators
An LED and a phototransistor in one package
current cannot pass from one side to the other.
Optoisolators
Some important parameters:
• Isolation voltage (typically thousands of Volts)
• Current Transfer Ratio (CTR = IC / IF × 100%)
• Speed (how fast can transistor turn on and off)
Voltage-Controlled Oscillators
Output frequency is proportional to input voltage.
VCO Applications
Some applications:
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Frequency modulator
Adjustable carrier-oscillator for a radio transmitter
Adjustable signal source
Analog-to-digital converter
Building block for Phase-Locked Loops (PLLs)
Phase-Locked Loops
Used in communications circuits.
PLLs
• The VCO is set to run at a center frequency.
• The VCO output is compared to the input in a phase
detector circuit. The bigger the phase difference
between the two frequencies, the higher the voltage
out of the phase detector.
• The output of the phase detector is fed through a LPF
and becomes the control signal for the VCO. That
closes the feedback loop.
• The VCO will eventually “lock on” to the input signal
and “track” it as the input frequency changes. The
VCO frequency will match the input frequency.
PLL as an FM Demodulator
PLL Frequency Synthesizer
f(out) = (n2 / n1 )  fXTAL