Current mirror with gain?

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previous one) :-)

It IS a feedback loop. More prone to oscillate if BJT, but CMOS can be coerced :-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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I love to cook with wine.     Sometimes I even put it in the food.

Unavoidable, in both circuits.

Sure, if it matters. For a backlight, probably not--you can barely detect/see/notice/discern a 30% brightness change, unless it changes quickly.

Yup.

-- Cheers, James Arthur

ed Bloggs

-

. . +5V . .----------. . | | . | | . [10k] | . | |< Q2 . +------- | BC557 . | |\ . | | . | .---------------. . | | 3,3V drop | . | | LCD backlight | . | '---------------' . | | . |/ | . 0-+5V >-[47k]-+------| Q1 | . | |> BC547B | . Pin 19 [5.1K] | | . PIC DAC out | +----------' . - | . v [15] . 1N914A - | . | | . /// ///

Where you have gain, feedback, and phase delay, there be dragons.

Let's look at it in the time domain. Suppose Q1 is slow and Q2 fast. Imagine a small impulse into Q1(b). Imagine Q1's collector current rising slowly, and i(Q2c)'s rise following very closely behind, applying negative feedback in lock-step to Q1's emitter. Stable.

Suppose the same impulse where Q1 is fast and Q2 slow. Time-wise, Q2's feedback signal to Q1 can't keep up. i(Q1c), lacking feedback, overshoots. Q2 finally responds. Q2,wound up, then applies too much feedback. And, continues to increase it even after Q1 is begging it not to. The output overshoots. Then, with Q1 overly starved (because the output's swung much too high), the process reverses and undershoots. Repeat. Unstable.

Mine screamed at 10's or 100's of MHz (I don't remember clearly).

That's one example. There are probably multiple possibilities.

-- Cheers, James Arthur

Fred Bloggs

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+5V

---.

=A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0|

=A0 =A0 =A0|

=A0 =A0 =A0|< =A0 Q2

| =A0BC557

=A0 =A0 =A0|\

=A0 =A0 =A0 =A0|

--------------.

3,3V drop =A0 =A0 |

LCD backlight |

--------------'

=A0 =A0 =A0 =A0|

=A0 =A0 =A0 |

=A0|

k e

Ummmm...are you serious? The circuit is dominated by only two poles , one of which is at really high frequency because of the scant 15R load. Loop phase shift is asymptotic to but never reaches 180o unless you have something else going on there.

On a sunny day (Wed, 28 Sep 2011 05:44:19 -0700 (PDT)) it happened snipped-for-privacy@yahoo.com wrote in :

Sure, that backlight is not critical. I tell you what, I realised today I can omit the voltage divider, as the PIC has a choice to set the reference of the DAC to its internal reference (stabilised too) of 1.024 V. This gives me 32 steps of 32 mV. So for 30 mA maximum that makes R2 = 1.024 / .03 ~ 33 Ohm. It is also nicely below the max voltage drop allowed, so plenty of voltage for the LCD backlight. I will try this later, now working on an other part of the project, This circuit now requires only 2 transistors and 2 resistors :-)

d Fred Bloggs

to

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you

=A0+5V

-----.

=A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0|

=A0 =A0 =A0|

=A0 =A0 =A0|< =A0 Q2

-- | =A0BC557

=A0 =A0 =A0|\

=A0 =A0 =A0 =A0|

.---------------.

| 3,3V drop =A0 =A0 |

| LCD backlight |

'---------------'

=A0 =A0 =A0 =A0|

=A0 =A0 =A0 |

=A0 =A0|

=A0|

ack

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use

Sorry, I lazily re-published your schematic for the topology, but wasn't talking about your implementation. I should've stripped out the component values.

I don't remember the combination that made my buffer scream way back when, only that it did.

-- Cheers, James Arthur

On a sunny day (Wed, 28 Sep 2011 05:44:19 -0700 (PDT)) it happened snipped-for-privacy@yahoo.com wrote in :

I tried it today wih 33 Ohm for R2 and 10k for R1, with 0 to + 1V input (the minimum I can get from the DAC). It starts up with ever so little current, but it becomes non-linear near 1 V input. Looks like I will need the voltage divider anyways, and smaller R2.

Or I could do this: +5 +5 | | LCD Rx | | c

0-5 V -- 22 k ----- b NPN e BC547B | /// That removes the deadband. Rx so at 0V input Ube is about 0.6V (guaranteed off). Makes Rx maybe 150 k.

Or you could use a mosfet, a current-limiting resistor, and PWM the LED for brightness control.

That's only two parts, and it's rock-solid.

-- Cheers, James Arthur

On a sunny day (Fri, 30 Sep 2011 04:12:36 -0700 (PDT)) it happened snipped-for-privacy@yahoo.com wrote in :

It is an idea, I am already using the {IC PWM of HV generation, and the interrupt for high priority things. But there is a timer tick at lower priority.... The timer tick is 5 ms, that would mean 5 mS on and 5 ms off for

50%, but 100 ms on and 5 ms off, for 5% on, would not be so nice I think. Or 100 ms off and 5 ms on, I am sure that would be visible flicker. I added the extra resistor, things work fine, 390 kOhm, as the DAC has a vey high output impedance. I like it, and that counts.