# BC337 transistor appears to be in active mode, but has too small Vbe

I've stumbled onto something that I don't understand.

I've wired up a phase shift oscillator using a BC337 transistor (datasheet here). According to the datasheet, $V_{BE(on)}=1.2\ V$ However, using my multimeter (and after doing the theoretical calculations), I have found that my $V_{BE}$ is about 0.6V, which should apparently force the transistor into cutoff.

Here is my circuit:

simulate this circuit – Schematic created using CircuitLab

The oscillator is functioning, producing a slightly distorted sine wave of about 1.5 V peak-to-peak and at about 560 Hz. I measured $I_E$ to be around 7 mA, but couldn't measure $I_B$ without my multimeter ruining the output of the circuit.

As far as I know, this output should indicate that the transistor is in active mode, but if $V_{BE}$ is that small, shouldn't it be in cutoff?

• The value shown as 1.2VDC in the datasheet is the "maximum" value of Vbe. As you might know, typical silicon transistors have about 0.6-0.7VDC, so your Vbe measurement is correct. But for Ie, if you measure 7mA then the voltage across R6 should be 7V, but the supply voltage is 5V. How can it be? If you make DC analysis then you can easily see that quiescent collector (or emitter) current is about (1.25-0.6)/1k=0.65mA. But since this is an oscillator, you might not able to measure DC currents. – Rohat Kılıç Dec 20 '16 at 4:20
• OK, guess I didn't know how to interpret that. In that case everything is fine. Thanks! – Mahkoe Dec 20 '16 at 4:29
• @Mahkoe Do you want an undistorted sine out? you never asked. – Sunnyskyguy EE75 Dec 22 '16 at 0:42
• @TonyStewart.EEsince'75 Haha thanks for asking. However, I figured that one out on my own. I reduced the gain of the amplifier (by putting the bypass capacitor on the slider of a potentiometer), and that almost completely removed any distortion. – Mahkoe Dec 23 '16 at 15:55
• Yes it has a narrow hFE gain window for linearity. There are many other ways to do it as well. You ought to be able to get >4Vpp keeping away from Vce(sat) and Vce(max (Imin>10%) – Sunnyskyguy EE75 Dec 23 '16 at 17:20