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Reverse Power Booster
Unusual Inverted Transistor Booster

The principle of the transistor is fairly simple; a non-conducting base material has added to it small amounts of impurities that alter the crystal structure (doping). This is accomplished by adding other elements such as phosphorus, antimony, gallium or indium into the layers of pure silicon. One type of impurity will make a surplus of conduction electrons (N-type) while another impurity will create a lack of electrons (P-type). This allows the transistor to conduct electrical flow under certain conditions (semi-conduct).

A transistor like the 2N5088 is made by preparing a wafer of N-material for the base. It is masked and etched in a series of steps which finally forms the three layers of the N-P-N structure of the transistor.

Diffusion of the impurities into each layer is carefully controlled. Epitaxial transistors formed by this photolithography method have improved properties for normal usage. Because the collector material completely surrounds the smaller emitter layer, and since the collector is a lightly doped region while the emitter is a heavily doped region, efficiency (forward beta) is improved and the reverse beta reduced.

An alloy-junction transistor is made through a different process. A base wafer of very pure material such as germanium is polished and cut to the desired size. It may contain a tiny amount of antimony to make it into an N-type material. Very small pellets of indium are placed on the wafer and when passed through a temperature controlled furnace, some of the metallic indium diffuses into the germanium wafer to make a P-type alloy infusion. The pellet used for the collector will typically be several times larger than the emitter pellet. Thin jumper wires are soldered to the indium pellets (collector and emitter) and the germanium substrate (base).

Examples of alloy-junction transistors are the OC44 and AC128. These devices have a much larger reverse beta than the typical epitaxial silicon transistors.

There are alloy junction silicon transistors such as the Raytheon CK793, but the diffusion mesa (2N497, 2N1047) and planar passivated epitaxial growth manufacturing processes soon made them obsolete.

This is a typical bipolar signal booster as used in guitar effects. Similar circuit layouts are found in every book on transistor design, and these component values are used in numerous boutique booster pedals. It is a basic common emitter circuit that offers plenty of gain while drawing little current.

With the components shown here, gain will be around +25db (more than 17x) when powered by a 9v battery. The beta of the transistor (2N5088) actually has very little impact on the amount of gain for this circuit.

While reading an article in Electronic Design about a transistor circuit for a radio receiver, I encountered the phrase "reverse beta". This immediately made me think that if a transistor can have reverse beta then it can have gain from a reverse configuration. This led me to the following circuit design.

In our novel circuit, the arrangement of the all components remains the same except the transistor has been flipped so that it is in an unusual operating mode. At first glance you might think this is an emitter follower but obviously it is not. It is an inverted transistor amplifier.

If you refer back to the first illustration of the transistor P-N layers, it can be seen that while the manufacture of the device has been optimized for operation in one direction, there is nothing that prevents us from using it backwards. Current flow is still from N-material to N-material with the base P-material modulating the flow.

Some component values have been altered to change the bias and make the circuit function properly. In this new configuration, the reverse transistor beta, which is quite low, has a direct impact on the circuit properties.

Measured gain of this setup on my breadboard was +12db (or 4x). On the oscilloscope, the boosted signal looked clean and pure.

What use does this circuit have? Well... the transistor will have more influence on the sound of the circuit so it might be possible to try different devices in this setup and get a variety of tonal flavors. It may be necessary to adjust the value of R1 when other transistors are used in place of the 2N5088.

However, the most interesting effect is when this reverse booster circuit is driven into clipping - it clips in a matter totally unlike that of the basic common emitter circuit! The overdriven reverse booster generates a mess of harmonics different from other clipping circuits and configurations.

The schematic to the left illustrates the use of an alloy-junction germanium transistor in reverse boost configuration. Alloy-junction transistors have a larger reverse beta and will give more gain. Note that this circuit uses a positive ground because the transistor is a PNP variety.

Once again, the value of R1 may have to be adjusted to account for the variations in individual transistors. For any of the circuits shown on this page, the DC emitter voltage should be one-half the supply voltage.

The Reverse Power Booster circuits shown on this page can be built on the Bipolar Booster pc board sold on this site.

There are plenty of ideas from this article to work with, and lots of new sounds left to be discovered, especially when the circuit is overdriven.

If you use this circuit design, remember to give credit and a link to this site. Thanks!

 

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©2006 Jack Orman
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First published on 22 NOV 2006
Last modified on Thursday, 13-Sep-2007 19:37:19 PDT

17850 hits since May 18, 2006

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