Hawes Amplifier Archive by James T. Hawes, AA9DT
Design a BJT Amplifier Without Using Curves

DIY Requires a Can-Do Attitude

Inventing isn't for everyone. Do-it-yourself requires a can-do attitude. The circuit doesn't always work. You get your hands dirty. You have to cipher with Ohm's Law: R= (V / I). It's hard, but this page shows you the way. You can make it if you try.

Prototype your amplifier on a solderless breadboard (plugboard). The circuit goes together in minutes. Changes are easy. Radio Shack and others sell such breadboards. When your invention is satisfactory, copy it to a perfboard or PC board.

Photo: Solderless breadboard (plugboard)
I found this breadboard on eBay. I has about 2,000 contacts. For a simple preamp, 400 to 800 contacts are enough. 		Art: Lightbulbs go on for young engineers.
Is it love, or a new circuit?


Prerequisites

  • NPN & PNP. This design process is suitable for NPN or PNP transistors. All examples refer to NPN devices, which are more common. To use a PNP part, reverse all power polarities. Also reverse all polar devices that connect to the transistor. (For example, electrolytic capacitors.) At right, note the schematic symbol for an NPN transistor. Mouse over for the PNP symbol.

  • Silicon. The device must be a silicon, bipolar junction transistor (BJT). (No FETs or MOSFETs. No Ge or GaAs devices.) Our method designs Class-A preamplifiers. We use the common-emitter circuit. A 2N3904, NPN transistor (right) will work fine with this method. PNP equivalent: 2N2907 or 2N3906.

 Schematic symbol for NPN transistor. Mouse over for PNP. Diagram: Generic 2N3904 in TO-92 package, showing pin layout. On mouseover, animation.
  • Bias. For proper operation, an NPN transistor base (B) must be 0.7 volts more positive than the emitter (E). We apply a DC bias voltage to the base. The positive bias turns the transistor on halfway, allowing current flow.

 Schematic: No emitter voltage from unbiased transistor. Mouse over for emitter voltage with proper base bias. Click for circuit with bias resistors.
Why zero? For the answer, mouse over. Click for circuit with resistor bias.

No-Curve Design Procedure

A—Find RE

  1. Pick the power voltage VCC. (Typical values are 3, 6, 9, 12, 15 or 18 volts.)

  2. Choose the average current IE for the circuit. (For a preamplifier, 1mA is typical.)

  3. Compute VE = (VCC / 10)

  4. Compute RE = (VE / IE).

♦ NOTICE. RE regulates the current through the transistor. The RE value sets the maximum IE.
Schematic: Calculate and add emitter resistor Mouse over to install RE in circuit.

B—Bias the Transistor with VBB

  1. Compute VBB= (VE + 0.7)

  2. Compute IBB= (IE / 10)

  3. Compute RB—total= (VCC / IBB)

  4. Compute RB2= (VBB / IBB)

  5. Compute RB1= (RB—total - RB2)

♦ NOTICE. The circuit that you have so far is an emitter follower. By itself, this circuit is useful as a buffer or power amplifier. Although the circuit produces no voltage gain, it does offer current gain. To use this circuit, take the output signal off the emitter. To achieve voltage gain, continue the design process.
Schematic: Bias resistors. Mouse over for resulting amplifier.
Mouse over for resulting circuit.

C—Complete the Amplifier

  1. Compute RC= (5 x RE).

♦ NOTICE. To reduce voltage gain, decrease the RC value. A smaller value reduces distortion, increases fidelity and broadens signal bandwidth. If RE exceeds RC, the circuit attenuates (is lossy).
Schematic: Install collector resistor. Mouse over for resulting amplifier.
Mouse over to install RC in preamp.

More About Amplifiers

Resistors come in standard sizes. Replace each calculated value with the nearest standard value. For example, your electronics store doesn't sell a 4,570-ohm resistor. Instead, buy the standard, 4,700-ohm part. In most cases, standard parts will be within 10 percent of your calculated values. If possible, the base-bias network should be even closer. Resistor wattage is important, too. For one-milliamp circuits, quarter-watt resistors are sufficient. Above 10 mA, switch to half-watt or larger resistors. If the resistors run warm, replace them with higher-wattage parts. In that case, also heat sink your transistor.

Decoupling Capacitor. To increase stage gain, you may add an emitter decoupling capacitor CE to the circuit. The costs of this capacitor are reduced stability, reduced low frequency response and gain that varies by frequency. The decoupling capacitor adds a high-pass filter to the preamplifier. For this reason, a full-band amplifier requires a very large emitter-bypass capacitor. For preamps, typical capacitor values vary between 50 µF and 1,000 µF. An amplifier with an emitter bypass capacitor can't amplify DC. If you make the bypass capacitor small, the preamplifier will only boost the high frequencies.

Coupling Capacitor. To achieve sufficient voltage gain, a typical amplifier requires more than one stage. Without a redesign, you can't directly connect two stages. If you do, the collector voltage of the first stage upsets the base bias of the second stage. Engineers devised ways to eliminate this problem. The easiest method avoids redesign by inserting coupling capacitors between stages. These capacitors block out the DC from the collector and prevent it from upsetting bias. Each transistor must have both an input and an output capacitor.

Typical values. A typical value of input coupling capacitor CB is 5 µF. A typical output capacitor CC is 50µF. As with decoupling capacitors, coupling capacitors add high-pass filters to your amplifier. Capacitor-coupled preamplifiers can't amplify DC or slow-changing AC signals. If the amplifier will pass low-frequency signals, only use large capacitors. For example, a bass guitar amplifier requires larger capacitors than does a banjo amplifier. Also use a capacitor after the pickup, at the input of the first amplifier stage.

 Schematic: Amplifier with coupling capacitors. Mouse over for amplifier with decoupling capacitor.
Coupling capacitors. Mouse over for CE (emitter decoupling capacitor ).

Computing capacitor values. A future lesson will discuss how to select coupling capacitors for your circuit. If you'd like to experiment, use the typical values in the paragraphs above.

Other Types of Amplifiers. Bias voltage is only necessary for analog circuits. A properly biased transistor can reproduce and amplify sine waves. Such sine waves make up natural sounds, such as popular or classical music. Without bias, an amplifier is possible, but the circuit will clip (distort) the sine waves. Some effect and switching circuits work without bias, because these circuits need not be faithful to the original waveform. A clipper circuit appears at right.

 Schematic: Clipper preamplifier for effects. Mouse over for typical part values.
Clipper circuit: Mouse over for typical part values.



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♦ 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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