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Prune  HeadWize Fanatic Joined: Nov. 18, 2002 |  Message [#1] posted on: 06-25-2005 03:33 AM CST (US).   |  |  I haven't found a good explanation on the web of what exactly a Vbe multiplier does. For example, looking at the Blue Hawaii in the project pages here, what does the Vbe multiplier do? [Edited by Prune on 06-25-2005 at 03:33 AM.] |
| rohitbd HeadWize Fanatic Joined: Aug. 27, 2001 | Message [#2] posted on: 06-25-2005 04:18 AM CST (US). | | A Vbe multiplier is another name for a constant voltage source. It is, in particular, a CVS that uses the Vbe as a reference to derive it's o/p voltage (Vce). By virtue of high Hfe and low base current, the Vce that appears across the transistor is given by - Vce = Vr1 + Vr2, where r1 is the resistor between C & B, and r2 is the resistor between B & E. Now, as long as the transistor is conducting, Vr2 = Vbe and since base current is negligible, Ir1 = Ir2 (= Ibb, say) Thus, Vce = Ibb(r1 + r2) and Ibb = Vbe/r2 substituting for Ibb, Vce = (Vbe/r2)(r1 + r2) hence, Vce = Vbe(1 + r1/r2). Since Vbe is constant for a given temperature, and Vce does not depend upon current, it follows that Vce is constant - which is also the o/p of the ckt and thus it works as a CVS. Of course in practice, temperature variations will cause Vce to change and this action (Vce changing with temp.) is used to make the Vbe mult. act as a "bias servo" in addition to deriving the bias voltage for the class-ab o/p stage. As long as Hfe is high enough and temp. fixed, Vce will only depend on Vbe and changes in Vbe (due to device tolerances) can be compensated for by making r1 (or r2) variable and adjusting it for the desired Vce. Once set, Vce will remain constant as long as the temp. doesn't change. [Edited by rohitbd on 06-25-2005 at 04:27 AM.] |
amb Headphone Council Joined: Apr. 1, 2004 | Message [#3] posted on: 06-25-2005 04:28 AM CST (US). | | A Vbe multiplier is a transistor plus a couple of resistors wired up such that the resistors form a voltage divider between the collector and emitter, feeding the base. The effect of this circuit is that the voltage that appears across the collector and emitter (Vce) is a multiple of the base-emitter voltage (Vbe, which, for a silicon transistor, is around 0.65V). The actual multiplication factor is determined by the resistor voltage divider ratio. If you google "Vbe multiplier" you get a lot of hits with useful info. The Vbe multiplier is most often used as a bias circuit for the output stage of an amplifier. There is often a trimpot to allow the Vce to be adjusted. The circuit could also be used just about anywhere you need a voltage difference (where voltage accuracy isn't too important). The Vce will vary a bit with respect to temperature due to the thermal coefficient of the transistor. |
| Prune HeadWize Fanatic Joined: Nov. 18, 2002 | Message [#4] posted on: 06-29-2005 07:32 PM CST (US). | | What about FETs? I see Vgs multipliers mentioned, but no clear explanation of what the advantages/disadvantages compared to using BJTs in this application are. Additionally, it is not clear to me what the purpose of the Vbe multiplier in the Blue Hawaii schematic is, and what the two 50K resistors on top of them, from the common base amplifier, do. [Edited by Prune on 07-08-2005 at 03:02 AM.] |
| Prune HeadWize Fanatic Joined: Nov. 18, 2002 | Message [#5] posted on: 07-08-2005 03:02 AM CST (US). | | Good, now that the site is back, I can get the answers I've been waiting for... |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#6] posted on: 07-09-2005 03:01 AM CST (US). | | In the blue hawaii, the Vbe multipliers are Q6 and Q7, and they provide bias voltage to the MOSFETs Q13 and Q14. You could build Vgs multipliers with FETs, but the variability from device to device means that it's difficult to predict what voltage you're going to get using fixed resistors. A trimpot will be a necessity to dial-in the desired voltage, which in most cases is just unnecessary nuissance. |
PRR Headphone Council Joined: Mar. 18, 2002 | Message [#7] posted on: 07-09-2005 03:58 AM CST (US). | | > Good, now that the site is back, I can get the answers I've been waiting for... rohitbd and amb answered the question you asked. I'm not sure you saw those. You immediately jumped to "FET"s, not specifying JFET or MOSFET. The key property of these schemes isn't quite the "fixed" voltage, but the fact that the voltage tends to vary the same way we need a bias voltage to vary. If you bias a BJT with 0.600V, it may flow 100mA now, but the current will soar when it gets hot and vanish when it gets cold. If you bias a second BJT with 10mA and Vbe-multiply about 1.1 times, you get a voltage that varies with temperature but tends to be the right voltage to keep your main BJT flowing 100mA over a wide temperature range. I have never seen a straight JFET Vgs multiplier. I can't say I have seen a MOSFET Vgs multiplier, but the use is obvious: to bias a MOSFET. The need is less because MOSFET temperture drift is less. So low that many folks get away with fixed-volt Zener bias. > in the Blue Hawaii schematic Would be nice, when asking about a project, to include the link so folks don't have to go rummaging in the Library. > what the purpose of the Vbe multiplier As amb said, to set the driver output about 16*Vbe above the negative supply rail to give an appropriate voltage for the next stage. Oddly, this uses a BJT Vbe to bias a MOSFET Vgs, not an apparent match. However it isn't really Vgs because of the 1K trimmer. Most of the voltage is dropped in the resistor. The net result is that Q13 Q14 current falls slightly as the amp gets hot. However I would also note that the Q6 Q7 stage isn't a pure Vbe multiplier. The base resistance seems awful high for exact multiplication. Beta drift will be important. Q13 and Q14 seem to have very high, nearly infinite, DC voltage gain. Frankly I don't see why it holds a set bias at all. Without more study, I must defer to Gilmore who does know what he is doing, though sometimes he does things other people can't follow without his depth of experience. > what the two 50K resistors on top of them... do. At first glance: nothing. In the greater context: they take some heat off of Q4 Q5, and they prevent disaster when things go wrong. This is NOT an easy circuit to analyze. It is the work of a master taking things to the limits. [Edited by PRR on 07-09-2005 at 04:02 AM.] |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#8] posted on: 07-10-2005 01:17 AM CST (US). | | Yeah, a Vgs multiplier can only be made with an enhancement mode MOSFET. JFETs, which are depletion mode, have negative Vgs in their normal operating range (for N channel) so it's not possible to make a Vgs multiplier from such a device. |
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