Download Chapter 14: Amplifiers & Oscillators

Chapter 14: Amplifiers &
Oscillators
Amplifiers: Overview
• Circuits which increase: voltage or current
– Take small input signal to reproduce output
waveform as larger amplitude
– Ie. circuits which provide gain
• Frequency selective (like a band-pass filter)
– Audio frequency
– Video (Cable / Satellite)
– Radio
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Design Criteria: Bias & Gain
• When designing or selecting an amplifier,
there are a couple of things to consider:
• Bias
– Recall three common types of bias:
• Class A, B, C
• Less common ones: Class AB1, AB2
• Gain
– The amount of output power compared to the
input power
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Bias: Class A
• Amplifies 360° of the waveform
– Provides the best linearity (least distortion)
• Less than 50% efficient
• Commonly referred to as a “linear amplifier”
• Generally the “PA (power amplifier) stage” of
a transmitter is a class-A circuit
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Bias: Class B
• Amplifies only 180° of the waveform
– Allows greater gain, but at the expense of linearity
• How do you deal with the missing portion of the
waveform?
– Push-pull circuits
• Two amplifiers operate in tandem, but 180° out of phase,
amplifying the whole signal
– Flywheel effect
• The “missing” 180° is reproduced by the circulating current
in an accompanying tuned circuit (recall the properties of a
parallel-LC cct.)
• 60-65% efficient
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Bias: Class C
• Amplifies only 120° of the waveform
– provides the greatest gain but also the least linear
bias
• Excellent for non-AM modes
– Can not be used with AM due to extreme nonlinearity (distortion of waves = distortion of
intelligence)
• ~75% efficient
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Gain: Powerout / Powerin
• Gain is the ratio between the input signal
power and the output of the amplifier
Power gain:
Gain (dB) = 10 log (Pout / Pin)
Voltage gain:
Gain (dB) = 20 log (Eout / Ein)
– this works for both Voltage and Current!
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Decibel Math: Review
Q: If an amplifier provides 36db(!!) gain, what would the
output power of an amplifier be if you were to “drive”
it with 1W of power?
– Recall:
• 3dB = 2x
• 10dB = 10x
– therefore count in groupings of 3db or 10db and multiply each of the
“grouping” values
• 36dB = 3 groups of 10dB plus 2 groups of 3dB
– 10 * 10 * 10 * 2 * 2
• Answer: 4000W !
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Linking Circuits: Coupling
• We often use simple wire to connect circuits,
however, sometimes it is necessary to isolate
AC circuits from DC influences
• It is possible to provide coupling of AC signals
while isolating DC currents
– Two common methods:
• Transformers
• Capacitors
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Filters: General Function
• Designed to couple circuits while excluding
unwanted energy from the circuit
– Pass or reject energy based on frequency
– Common types of filters are:
•
•
•
•
Low pass
High Pass
Band Pass
Band Stop
• Used extensively through out electronics
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Filters: Low-Pass
• Allows low frequencies to pass with minimal
attenuation while blocking all frequencies
above the cut-off frequency
• Often used between TXVR and antenna
f = low
high
Low frequency
passed on to RL
High frequency
diverted thru C
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Filters: High-Pass
• Allows high frequencies to pass with minimal
attenuation while blocking all frequencies
below the cut-off frequency
• Used to be common as TVI filters
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Filters: Band-Pass
• Allows a range of frequencies to pass,
rejecting those above and below the cut-off
frequencies
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Filters: Band-Stop
• Block a range of frequencies
– In radio terms, often knows as a “notch filter”
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Filters: π
• Many low-pass filters use what is known as a Pifilter configuration
• As you increase the number of π-elements, you
improve the filter’s selectivity (performance)
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Decoupling
• Decoupling is used where components are
sensitive to change in the input signal
• Often called a buffering stage
• Provides the ability to isolate circuits
– Prevents DC current on AC signal line
• Most common methods
– Capacitors
– Transformers
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Increasing Gain: Cascading
• Vacuum tubes and transistors can only provide
so much gain
– Vacuum tubes provide voltage gain
– Transistors provide current gain
• To increase total gain of an amplifier, it is often
necessary to have multiple amplification
stages linked in series
– This is known as cascading
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Feedback: Can be useful
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