AI is the current gain of the amplifier circuit. Typical form is IIN / IOUT (with IOUT = 1 amp). Alpha (α) is the ratio (IC / IE). That is, how much collector current comes from the emitter. This is a direct current relationship. Typically, the ratio approaches 1. Formula: α = (β / (β + 1) ) AP is the power gain of the amplifier circuit. Typical form is PIN / POUT (with POUT = 1 watt). AV is the voltage gain of the amplifier circuit. Typical form is VIN / VOUT (with VOUT = 1 volt). Beta (β). Current gain of the raw transistor: How much base current produces how much emitter current. Beta is a static or DC measurement. Beta increases with IE and with heating. Formula: β = (α / (1 - α) ). Same as HFE.

BJT is an abbreviation for “bipolar junction transistor.” The device is bipolar because both electrons and holes (carriers) flow through it. BJTs are normally-off devices. Carrier refers to an electron or hole. In a transistor, electrons flow in one direction, and holes flow the opposite way. There are majority and minority carriers. For instance, in an NPN transistor, electron current flow is greater than hole flow. For that reason, electrons are the majority carrier. In PNP transistors, holes are the majority carrier. CB is the input capacitor that connects the base to the input circuit or transducer. CC is the output capacitor that connects the collector to the output circuit or transducer. CE is the bypass capacitor across the emitter. It allows for increased gain and produces a high-pass characteristic. To increase the low frequencies, increase the size of this capacitor.

Coupling capacitors. Mouse over for decoupling capacitor.

Class A amplifiers are always on. They can amplify both the positive and negative excursions of a sine wave. The fidelity of the Class-A amplifier is the highest among all amplifier classes. The power efficiency of the Class-A preamplifier is the lowest. Fidelity is why Class-A circuits are desirable for preamplifier and low-power amplifiers. Typically, Class-A circuits aren't so desirable for high-power amplifier circuits. Big Class-A circuits run very hot. Common Emitter Circuit. In our design method, we consider the common-emitter (CE) circuit. In this circuit, the common element is the grounded emitter. The signal enters the base. The output arrives at the collector. The base controls current flow between the emitter and collector. The bias causes the collector to operate at about half the power supply voltage. We read voltages between any electrode (C/B/E) and ground.

NPN CE preamp. Mouse over for typical values.

FET. Field-effect (unipolar) transistor. Depletion FETs are normally-on devices, like tubes. Enhancement FETs are normally-off devices, like BJTs. Zero-threshold FETs require no bias. FET threshold (bias) voltages differ from BJT threshold voltages. For this reason, our method can't design FET circuits. FET advantages: High impedance. Extended low and high frequency reproduction. Low noise. Simpler circuits. No thermal runaway. Disadvantages: Low gain (10 percent of BJT gain). Few power devices. Wide spec spread. Vulnerability to static. See MOSFET. Right: N-channel FET symbol. Mouse over for P-channel FET symbol.

• F_T is the top frequency of the amplifier. The top frequency relates to the bandwidth. See GBP.

• GBP. Gain-bandwidth product. Doubling the gain halves the amplifier's top frequency. To increase the top frequency, decrease the gain. The GBP of a typical, general-purpose transistor is about 300 MHz. Our design method works reliably up to 10 MHz. Above that, current gain falls below 30. The designer must compensate by increasing current through the base bias resistors.

• GaAs. Gallium arsenide, a semiconductor material. GaAs transistors have a threshold (bias) voltage of 1.5 volts. GaAs BJTs are uncommon. Designers prefer silicon (Si), because it is cheaper and widely available. Si also operates at lower voltages. Advantages of GaAs: Low noise. High frequency. Same bias voltage as LEDs. GaAs junctions can emit light.

• Ge. Germanium, a semiconductor material. Ge transistors have a threshold (bias) voltage of 0.3 volts. Once common, these devices are rare today. Designers prefer silicon (Si), because it is less leaky and less prone to thermal runaway. Si devices also produce far more gain (10X). Advantages of Ge: Ge “leakage bias” requires no bias resistors. Operates at very low voltages. Replacement parts for antique equipment.

• H-Parameters are the four-pole, hybrid characteristics of transistors. Transistor datasheets use these small signal AC parameters to specify a transistor's dynamic (AC) performance and behavior. Despite theoretical specs, devices of one type vary considerably. Typical h-parameters include h_fe, h_ie and h_oe, and h_re. Subscript “f” indicates a forward measurement, and “r” indicates reverse.

• H_FE is the current gain of the raw transistor in a common-emitter circuit. That is, how much current into the base produces how much current out of the emitter. The H_FE formula is (I_C / I_B). Often, H_FE is a static or DC measurement (despite its being a dynamic h-parameter). Same as beta (usually).

• H_fe is the small-signal current gain of the raw transistor in a common-emitter circuit. That is, how much base current produces how much emitter current. This is a dynamic or AC measurement. H_fe is particularly important because it varies with frequency.

• H_ie Short circuit input impedance. A hybrid parameter.

• H_oe Open circuit output admittance. A hybrid parameter.

• H_re Open circuit reverse voltage transfer ratio. A hybrid parameter.

IB is the base current necessary for transistor operation. IBB is the current in the base voltage divider circuit. This value includes IB, but should be much greater to insure stability. The recommended minimum value is five percent of the IE value. This value allows for maximum stable current gain. A typical IBB value is ten percent of IE. IC is the collector current. A typical value for a preamp is 1 mA. High-impedance preamps such as some transducer amps operate at smaller currents. IE is the emitter current, which is the sum of base and collector current. The IE value sets the maximum current for the transistor circuit. MOSFET. Metal oxide field-effect (unipolar) transistor. May be normally-on or normally-off devices. MOSFET threshold (bias) voltages differ from BJT threshold voltages. For this reason, our method can't design MOSFET circuits. Advantages of MOSFETs: Even higher impedance than FETs. Small size, ideal for integration. Rapid switching. Low drive current. Disadvantages: Low gain. Power devices can't handle as much current as BJTs. Wide spec spread. Extreme vulnerability to static. See FET.

IE (emitter current). Mouse over for IC (collector current). Click for IB (base current).

NPN, PNP. Bipolar transistors are available in two types or “sexes,” NPN and PNP. A bipolar transistor usually has three legs: Emitter (E), collector (C) and base (B). Each leg corresponds to one terminal of the device. In a given circuit, any of the legs may be the input or output. The third leg typically becomes the “common” terminal, because it connects to the common reference. With NPN transistors, the common reference is usually ground. At right, note the schematic symbol for an NPN transistor. Mouse over for the PNP symbol.

• Ohm's Law. To find the resistor values for our circuit, we use Ohm's Law. Ohm's Law is a simple ratio of voltage E to current I. The result appears as resistance R.

  Ohm's Law: (E / I) = R
  
  Note. In our design method, we replace E with V, as is customary today.

RB1 is the top base resistor that connects to VCC. RB2 is the bottom base resistor that connects to VEE (typically ground for NPN transistors). RB—total is the sum of the two base resistor values RB1 and RB2. RE is the emitter resistor. This resistor sets the emitter current. Typically, the emitter resistor also improves the stage fidelity through negative resistance. REI is the internal emitter resistance. REI = (26 / IE), with IE in mA. RC is the resistor between the collector and the power rail. Increasing the value of this resistor up to five times the RE value increases the voltage gain. Si. Silicon, a semiconductor material. Si transistors have a threshold (bias) voltage of 0.7 volts. In most applications, the industry prefers Si devices over other types. Advantages of Si: Cheap and widely available. High output (up to 10 times that for Ge devices). Stable and dependable. Predictable. Easy to use. Handles current well. Good fidelity.

NPN amplifier with resistor names. Mouse over for PNP version.

VB is the necessary base bias to turn the transistor on halfway, initiating Class-A operation. VBB is the power voltage (potential) for the base circuit. Usually this voltage derives from a voltage divider in the base circuit. On equivalent (theoretical) circuits, the base supply often appears as a separate battery. VE is the emitter voltage. This voltage is the drop across the emitter resistor. For example, let IE = 1 mA and RE= 1K. Then: IE = (RE x IE) = 1 volt (by Ohm's Law). VC is the voltage between collector and common (typically ground, in NPN circuits). Vcc is the power voltage (potential) for the collector circuit. VEE is the power voltage (potential) for the emitter circuit. For NPN circuits, this potential is often ground (zero volts).

With reference to ground, voltages above are positive. Mouse over for PNP version.

♦ WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

♦ WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster

Copyright © 2012 by James T. Hawes. All rights reserved.

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