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 2

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 3

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 4

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 5

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 non- linearity (distortion of waves = distortion of intelligence) ~75% efficient 6

Gain: Power out / Power in Gain is the ratio between the input signal power and the output of the amplifier Power gain: Gain (dB) = 10 log (P out / P in ) Voltage gain: Gain (dB) = 20 log (E out / E in ) – this works for both Voltage and Current! 7

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 = – 10 * 10 * 10 Answer: 4000W ! plus 2 groups of 3dB * 2 3 groups of 10dB 8

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 9

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 10

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 = lowLow frequency passed on to R L f = high High frequency diverted thru C 11

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 12

Filters: Band-Pass Allows a range of frequencies to pass, rejecting those above and below the cut-off frequencies 13

Filters: Band-Stop Block a range of frequencies – In radio terms, often knows as a “notch filter” 14

Filters: π Many low-pass filters use what is known as a Pi- filter configuration As you increase the number of π-elements, you improve the filter’s selectivity (performance) 15

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 16

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 17

Feedback: Can be useful 18