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Emitter follower as current booster

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audioguru

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I HATE texting the same things on two websites. It is like a BIG BAD ECHO!
 

Roff

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Yeah, here's what I posted on AAC.
I tested the circuit with a lousy old LM324 made by TI. I used a 33Ω load. The output slewed from 0 to 5V in 16usec, and from 5V to 0V in 16usec.
I'm not saying that all LM324s will slew this fast. I'm just sayin'.
 
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audioguru

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National Semi made all the old LM324 opamps. TI started making them recently. Maybe TI ones are better than the old ones. But their datasheets are the same as the old ones.
 

Roff

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National Semi made all the old LM324 opamps. TI started making them recently. Maybe TI ones are better than the old ones. But their datasheets are the same as the old ones.
Actually, 5V/16usec is only about 0.3V/usec, which is reasonably close to the graphs in the TI datasheet.
My point was (and still is), even if the transition times are 100usec, that is probably OK for the OP's 1ms pulse.

5v/100usec=0.05v/usec.
 

vinodquilon

Member
fine

I have implemented 1ms 555 monoshot and cascaded it to the (Resistor ladder + Op-Amp_BJT) to form the final project.

Wow! everything is working fine as expected.:)

But here is one interesting factor I have noticed, without any trigger 555 monoshot outputs 10mV. Is there any noise effects?:mad:
 

vinodquilon

Member
Output

Here is my output for the circuit implemented using 555-LM324-2N2222 with Connection taken from 4.5V plug of the resister ladder.

Anomalies observed:

1. Even if I have implemented one Differentiator circuit, according to the manual pushing speed of the trigger switch the 1ms output duration is 'slightly' varying and some disturbances occur in the output pulse

Can I solve this by incorporating 74LS14 schmitt trigger inverters(2 Nos) in series with Microswitch.

2. Some noise like spikes are there at high level and low level of the output pulse. Is this is due to circuit or measurement setup ?

(0.5V voltage divisions from 5 to 0V are perfectly taking place with the use of resistor ladder having resistors of tolerance 0.01%)
 

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audioguru

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Don't forget that a switch has contact bounce. When the switch is turned on then its contacts bounce and make and break over and over creating many pulses. A contact debounce circuit might be needed.
 

audioguru

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I am using non-electrolytic capacitor as timing capacitor connected to pin 7 of 555. Is it OK?
You forgot to say WHAT KIND oF CAPACITOR!!
If it is a film capacitor it is fine.
But if it is ceramic then the frequency will probably change when the temperature changes.
 

vinodquilon

Member
Don't forget that a switch has contact bounce. When the switch is turned on then its contacts bounce and make and break over and over creating many pulses. A contact debounce circuit might be needed.
Do you mean this type of arrangement with low values of Capacitor chosen...
 

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vinodquilon

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Thanks Roff, I will check the performance of debouncer circuit and report here.


We have disused about the slew rate and rail in/out capacity of the op-amp to sync with 1ms pulses ranging from 0-5V in 0.5V steps. Can 2N2222 power transistor sync with voltage levels and timings ? Is there any better options ?
 

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vinodquilon

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I dont want to connect a capacitor across a switch directly as the switch shorts out the cap when it is closed. That's not that good of an idea. However, with a small resistor like 100 ohms in series with the cap, all is well.

If I connect 100 Ohm in series with the capacitor across the switch, will it affect the Lower Threshold Point of Schmitt trigger inverter ?
 

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ericgibbs

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hi,
Your circuit shows a +10V supply to the 74LS14, check the IC's datasheet,
E.
 

vinodquilon

Member
Actually my 555 output at 10 V is greater than 5V and I used 50K POT to adjust it to the required level at load. For example take output from 4.5V leg of ladder and adjust the 50K POT to get 4.5V across the load. Without disturbing the set value of POT take output at all other voltage positions. One time adjustment is applicable for all voltage levels.

If I have exact 5V at top of ladder then this one time 50K adjustment is not required. For that I can use the cases you said, or 74121 / 74123 ICs as they can output exact 5V.

If I have exact 5V at top of ladder(by using 74121 instead of 555) then resistors of the ladder should be have 0.01% tolerance levels for equal divisions of this 5V.
My doubt is then the 50K in parallel with them could change the balanced condition of the ladder. Thus no equal voltage divisions takes place. Can I avoid this by placing 100K or 1M ohms (>> 1K ladder) in place of 50K without disturbing both ladder and the op-amp.

If I avoid POT and connect op-amp directly to ladder , then floating condition would be resulted between switching intervals of ladder outs.
 

Roff

Well-Known Member
No , Schmitt NOT have additional 7805 drive not shown in the figure.
the 555 requires a trigger pulse that goes below vcc/3. With a 10V supply, the pulse needs to be at least ≈7V p-p. You can't get that with the output of a 74LS14, which is only about 4V p-p. You need to use a CD40106, with the same vcc as the 555.
 

audioguru

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The output high voltage of an ordinary 555 changes when the temperature changes.
An MM74C14, 74C14 or MC14584 can replace a CD40106 IC.
 

vinodquilon

Member
the 555 requires a trigger pulse that goes below vcc/3. With a 10V supply, the pulse needs to be at least ≈7V p-p. You can't get that with the output of a 74LS14, which is only about 4V p-p. You need to use a CD40106, with the same vcc as the 555.
As shown below, the output of the differentiator is only -ve going spikes which are already less than Vcc/3. Hence 4Vpp wouldn't be a problem.
 

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