Physics 340
Audio Engineering

Review Sheet For Exam #3:

Some of the things you should know for the third exam:

Vacuum Tubes:

Understand the principle of operation of the triode vacuum tube
Thermionic emission: electrons are "boiled" off of the cathode
Cathode: usually oxide coated, emits electrons
Plate produces an electric field to accelerate electrons away from the cathode
Grid produces electric field that opposes that of the plate. Controls electron flow
Grid is physically closer to the cathode. Smaller potential difference produces a large field.
Understand amplification factor, plate resistance, and mutual conductance and the mathematical relationship between the three quantities
Amplification factor and how we measured it: change in plate voltage divided by change in grid voltage while keeping the plate current constant
Plate resistance
Mutual conductance and how it is measured
Mutual conductance is amplification factor divided by plate resistance
Mutual conductance also equals change in plate current divided by change in grid voltage. It is the slope of the plate current versus grid voltage graph.
Be able to find the mutual conductance given a graph of plate current versus grid voltage
Understand the basic common cathode circuit and how we used it to measure mutual conductance
Common cathode circuit is an inverting circuit because plate voltage drops as grid voltage becomes less negative
Change in plate voltage is change in plate current times the load resistance
Plate voltage is plate supply voltage less the voltage drop across the plate load resistor

Bipolar Transistors:

Understand the principle of operation of the bipolar junction transistor
Wafer of three materials: NPN or PNP
Electrons and holes
Reverse bias: high junction potential
Forward bias: low junction potential
Base is very thin compared to collector and emitter
Base-emitter junction is normally forward biased
Base-collector junction is normally reverse biased
Most of the majority charge carriers from the emitter travel across the base into the collector
Base current very small compared to collector current
Results in current amplification
Forward current transfer ratio and how we measured it: change in collector current divided by change in base current. It is the slope of collector current versus base current graph.
Be able to find the forward current transfer ratio given a graph of collector current versus base current
Understand the basic common emiiter circuit and how we used it to measure forward current transfer ratio.
Common emitter circuit is an inverting circuit because collector voltage drops base current is increased.
Change in collector voltage is change in collector current times the load resistance
Collector voltage is collector supply voltage less the voltage drop across the collector load resistor

Microphone Transducers:

Convert sound vibrations in the air to an electrical signal
Four basic type: carbon, ceramic/crystal, dynamic, condenser
Carbon
Resistance changes as packing of carbon granuals is altered by diaphram
Low impedance
Requires power source
Very rugged
High output
Low fidelity - poor high frequency response
Cheap
Common in telephones
Ceramic/Crystal
Relies on piezoelectric effect: sheer distortion of certain crystals/ceramics produces a potential difference
High impedance
Requires no external power source
Very rugged
High output
Better fidelity than carbon, but high frequency response still not as large as other types
Cheap
Dynamic
Relies on electromagnetic induction to "generate" its output. Moving conductor in a magnetic field produces an induced EMF. Two type: moving coil and ribbon microphone
Moving coil: small coil actuated by a diaphragm moves in magnetic field
Low impedance, but may drive a transformer to convert to high impedance
Requires no external power source
Rugged
Low output: requires more amplification than carbon or ceramic/crystal
Very good fidelity. Can have a very wide frequency response.
Transient response only moderate
More expensive than carbon/ceramic types, but not as expensive as ribbon or condenser types
Very popular and widely used in audio. Probably the most common type.
Ribbon: delicate ribbon directly actuated by sound pressure variation moves in magnetic field
Low impedance, but may drive a transformer to convert to high impedance
Requires no external power source
Low output: requires more amplification than moving coil, carbon, or ceramic/crystal
Excellent fidelity. Has very wide frequency response.
Transient response excellent
Very delicate. Never use one in a bar or where it won't be taken care of.
Expensive.
Inherently operates on pressure gradient only: pattern is bi-directional figure-8
Used in audio by "those in the know" who can afford and take care of them.
Condenser
Relies on change in voltage across a capacitor as the capacitance is changed by sound pressure variations
Very high impedance
Externally polarized types require an external power source to polarize the capacitor, but electret types have a permanently polarized capacitor element
Not as rugged as dynamic moving coil microphone
Very low output. Requires a preamplifier in the microphone to boost the output to an acceptable level.
Preamplifier requires an internal or external power source
Excellent fidelity. The best possible
Excellent transient response
Widely used in recording studios
Expensive. Switchable pattern types can cost thousands.

Microphone Directional Characteristics:

Allowing sound to reach the back of the transducer results in a directional response
Directional microphone can reject unwanted sound
Better isolation
Less chance of feedback
Response is given by a mathematical polar equation
Polar equation is for amplitude, not intensity
Output voltage of microphone is proportional to the polar equation
Various patterns are possible:
Omni-directional: senses pressure only
Bi-directional or figure-8: senses pressure gradient only
Other types fall in-between: cardioid, hypercardiod, supercardioid
Be able to:
Use polar equation to find levels off axis
Use polar equation to find angle at which zero output occurrs
Combine polar equations for two microphones to find the resulting polar equation/directional response