Using a boost transistor to obtain power

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Using a boost transistor to obtain power [Copy link]

We all know a principle: the power output by a transistor varies with the output voltage. The higher the voltage, the greater the power .

The transistor works by the movement of carriers. Taking the npn emitter follower as an example, when no voltage is applied to the base, the pn junction composed of the base and emitter regions prevents the diffusion movement of majority carriers (holes in the base region and electrons in the emitter region). An electrostatic field (i.e., a built-in electric field) pointing from the emitter region to the base region will be induced at this pn junction. When the direction of the applied positive voltage to the base is from the base region to the emitter region, and when the electric field generated by the applied voltage to the base is greater than the built-in electric field, the carriers (electrons) in the base region will have the possibility to flow from the base region to the emitter region. The minimum value of this voltage is the forward conduction voltage of the pn junction (generally considered to be 0.7V in engineering).

However, there will be charges on both sides of each pn junction at this time. If a positive voltage is applied to the collector-emitter, under the action of the electric field, the electrons in the emitter region move to the base region (in fact, the electrons move in the opposite direction). Since the width of the base region is very small, the electrons can easily cross the base region to reach the collector region and recombine with the PN holes here (close to the collector). To maintain balance, the electrons in the collector region are accelerated to move to the outer collector under the action of the positive electric field, while the holes move to the pn junction. This process is similar to an avalanche process.

The electrons in the collector return to the emitter through the power supply, which is the working principle of the transistor. When the transistor is working, both pn junctions will induce charges. When the switch tube is in the on state, the transistor is in the saturation state. If the transistor is cut off at this time, the charge induced by the pn junction needs to return to the equilibrium state, and this process takes time.

Below we will introduce an example of how to use the boost triode to get more power. Although the 6L6 beam power tube used has been around for 66 years, it is still very popular in electric guitar amplifiers. Its similar 6CA7 (EL34) power pentode is also loved by hi-fi audio "enthusiasts".

The developers of these tubes designed them to operate in pentode mode, where they are able to deliver maximum audio power. On the other hand, many audiophiles prefer to operate in triode mode, and until now have had to reduce output power by 50%. The reduced output power means that they need a larger power supply and twice as many expensive tubes to get pentode power out of a triode amplifier. Figures 1a, 1b, and 1c show three ways to connect the 6L6 to a pentode, a real triode, and a "boosted triode," respectively. The boosted triode configuration allows the pentode to produce pentode-like power when operating in true triode mode. To understand the operation of a boosted triode, it is helpful to review vacuum tube theory. The 6L6 is a beaming power tube with a cathode, control grid, screen grid, suppressor grid, and anode. The suppressor grid is actually a virtual suppressor grid provided by two bunching plates, but the 6L6 beaming power tube can be treated as a pentode. You can think of the pentode as an n-channel JFET with the following electrode functions:

Figure 1. A pentode (a) can output much more power than a triode (b), except when using a boost triode configuration (c).

* Hot electron cathode: electron source (corresponding to the JFET source);

* Control gate: controls cathode current; operates at a negative potential relative to the cathode (corresponding to the JFET gate);

* Screen grid: Electrostatically shields the control grid and plate, thereby reducing the effect of anode voltage on cathode current; operates at a positive potential relative to the cathode;

* Suppression grid: prevents secondary electrons from leaving the anode and reaching the screen grid; operates at cathode potential;

* Anode: collects electrons (corresponds to the JFET drain).

Figure 2 The load curve of the pentode shows that the anode can absorb 150 mA when the anode voltage is only 50V.

Figure 2 above shows the pentode characteristic curve when the control grid voltage is 0~ -25V and the screen grid voltage is 250V. Please note the idealized load line and that the pentode can absorb 150 mA of anode current when the plate voltage is only 50V. High voltage gain, high anode impedance and high output power are the three characteristics of pentode amplification. As long as the screen grid is directly connected to the anode, the pentode can be operated in triode mode. Low voltage gain and low output impedance are the characteristics of this mode.

Figure 3 A pure triode requires 200V anode voltage to absorb 150 mA current

Figure 3 above shows the difference between the triode and pentode curves. These curves represent control grid voltages from 0 to -90V. Note the load line and the fact that the plate cannot sink 150 mA below 200V in triode mode. This fact greatly limits amplifier efficiency and output power. However, despite the limited output power, some people prefer triode mode because they claim it produces a superior sound amplifier.

Figure 4 A 100V screen grid power supply can transform a normal transistor into a boost transistor.

Figure 5 The anode of a boost transistor can absorb 150 mA at an anode voltage of 100 V, while a pure transistor requires an anode voltage of 200 V.

For the boosted triode circuit shown in Figure 1c, you simply add a 100V screen grid-anode supply to the standard triode amplifier circuit (Figure 4). This shifts the triode characteristic curve 100V to the left (Figure 5).

Note that the load line and plate can now draw 150 mA at an anode voltage of only 100 V instead of the 200 V required for a pure triode mode circuit. You can get much more power out of boosted triode amplification and still retain the characteristics of triode amplification. In a Spice simulation of three single-ended Class A audio amplifiers using MicroCap-7 evaluation software, the control gate bias for the quiescent anode current is 75 mA, and the AC gate signal is just short of amplifier limiting. The transformation ratio provides an anode load impedance of 5 kΩ for the pentode and 3 kΩ for the triode and boosted triode.

In short, in the switch circuit, when the transistor is turned on, Uce reaches the minimum value, and the current reaches the maximum value at this time; when the transistor is turned off, Uce reaches the maximum value, but the current is almost 0. Therefore, in the switch circuit, the power dissipation of the transistor is very small in the "on" and "off" states, and in the conversion process of "on" and "off", the values of Uce and Ic are relatively large, and the power borne by the transistor is relatively large. When the collector current passes through the transistor, heat will be generated. The maximum collector dissipation power when the parameter change caused by the heat does not exceed the allowable value is called PCM. The actual dissipation power of the tube is the product of the collector DC voltage and current, that is, Pc=Uce×Ic. Use Pc when using it.