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### Topic: Transistor Graphing Device (Read 5240 times)previous topic - next topic

#### DROBNJAK

##### Sep 30, 2013, 12:09 pmLast Edit: Sep 30, 2013, 12:20 pm by DROBNJAK Reason: 1
I am grapling with a design of the common emitter biased transistor amplifier. Just want to go beyound Arduino and learn a proper, analog stuff.

Anyway, the circit I made suffers from clipping, no matter what I do. And I want to get a nice sinusoid from top to bottom. So I thought of getting one of these devices that can graph transistors' curves. Unfortunately, although device was brilliant at what it did, it was only designed for very low currents, maybe 50mA max. And my circuit works all the way up to 500mA, so it is out of the range of this device.

Now, as far as I can get it, for transistor graphing device I need some form of resistance that I can vary digitally, for example via commands sent from Arduino. Unfortunatelly, all the digital pots I can find can only handle less than 10mA, if that. Practically there is no digitally controlled component that can stand currents in full range from 10-1,000mA.

So I looked at resistor networks and maybe a shifter IC. I thought I can use shifter IC (like it is used for switching LED segments) to select parts of resistor network and create digitaly controlled high current voltage devider. Again, most of resistor networks I found have less than 5-10 legs, while I would need much more than that in order to acheive finer granularity for curves.

As a last resort I thought connecting servo mottor to the pot, but definitelly that would be an overkill.

Any ideas?

#### MarkT

#1
##### Sep 30, 2013, 12:16 pm
The name is "curve-tracer".

You don't want a programmable resistor, you want a programmable current source/sink.  This is
a simple opamp circuit - for higher currents you'd add a big transistor in the loop to boost output
levels.  Use a DAC to generate the programming voltage for the current source, and measure the

Here's a circuit than sources current (a bit harder than a programmable sink)
http://www.seekic.com/circuit_diagram/Power_Supply_Circuit/VOLTAGE_PROGRAMMABLE_CURRENT_SOURCE.html
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### DROBNJAK

#2
##### Sep 30, 2013, 12:23 pmLast Edit: Sep 30, 2013, 01:54 pm by DROBNJAK Reason: 1
;-))) programmable resitor. I am such a rookie. Thanks.

By the way, does anybody know any comercial transistor curve tracer that can work in 10-1,000mA and is bellow \$200?

#### polymorph

#3
##### Sep 30, 2013, 04:38 pm
Quote
By the way, does anybody know any comercial transistor curve tracer that can work in 10-1,000mA and is bellow \$200?

Yeah, the one you are going to build.

You don't really need that. Can you show us the schematic? There are a lot of things to take into account. It is not really possible to get true symmetrical clipping with a single transistor Class A amplfier. You might consider a Class AB push-pull amplifier.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### KeithRB

#4
##### Sep 30, 2013, 05:30 pm
Heathkit used to have one about 30 years ago...

#### polymorph

#5
##### Sep 30, 2013, 06:47 pm
http://forum.arduino.cc/index.php?topic=70003.0

http://www.idea2ic.com/BiCmosCurveTracer/Arduino%2520BiCmos%2520Curve%2520Tracer.html

Both low current, but it gives you a place to start.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### DROBNJAK

#6
##### Sep 30, 2013, 07:31 pm
I have this one:

http://www.peakelec.co.uk/acatalog/dca75-dca-pro.html

but it is puny little thing, only goes to 10mA. For an ordinary BC548 transistor, that is about 1/10th of its capacity. It has brilliant software, though.

#### polymorph

#7
##### Sep 30, 2013, 09:04 pm
In any case, this will be of limited utility for the purpose. I'd very much like to see a schematic.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### DROBNJAK

#8
##### Sep 30, 2013, 10:10 pmLast Edit: Oct 01, 2013, 06:57 am by DROBNJAK Reason: 1
Here is the circuit. Pure marvel of technology

Values on the picture are wrong. Real values are these:

Vcc= 16.5VDC, RIN= 33?, CIN= 4.73?F, R2= 8.97k?, R1= 1.253k?, Rb= 0.00?, Rc= 167?, Re= 13.4?, Clow-f= 47.7?F

#### polymorph

#9
##### Sep 30, 2013, 11:11 pm
Why are there values expressed as fractions?

Could you just get out a pencil and paper, and draw a schematic with the correct values written by the components?

I think you are chasing a unicorn, trying to get symmetrical clipping in a single transistor Class A amplfier.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### DROBNJAK

#10
##### Sep 30, 2013, 11:26 pm

Why are there values expressed as fractions?

I think you are chasing a unicorn, trying to get symmetrical clipping in a single transistor Class A amplfier.

Sorry about fractions. I removed them. That was just error. Please have a look again.

Are you saying that single transistor Class A amplifier will always clip on one side? That means essentially my design work "normaly" for what it is.

Strange thing is that I had these same values on a breadboard and it worked without clipping. Only when I transfered the design onto perfboard it started clipping.

#### polymorph

#11
##### Oct 01, 2013, 12:13 am
So, roughly 100mA bias point (about 2V on the base, about 1.4V at RE). Across a 167 ohm collector resistor is 16.7V. However, that is 2V fed through a thevenin resistance of about 1k, and so the additional drop due to base current depends on the transistor beta. A rule of thumb is that the base bias network should have about 1/10th of the collector current flowing through it.

Or am I getting the right values? If you want help, it is better to not make things difficult for those trying to help you. Could you repost a schematic  with the correct values on the schematic?

I'm saying a class A amplifier driven to clipping will always clip assymetrically. The output impedance changes with the bias point, so when it is near cut-off, output impedance is higher, when near saturation, output impedance is lower.

If you want better linearity, use a Class AB Push Pull amplifier with negative feedback.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### polymorph

#12
##### Oct 01, 2013, 12:24 am
I would also like to say that I hope you do not find anything I say to be discouraging. I applaud you for learning more about electronics. So many stop with plugging in shields.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8

#### DROBNJAK

#13
##### Oct 01, 2013, 07:04 am
Hi, I uploaded a new image with clear values.

Yeah, learning the analogue side was like jumping on hyper-drive. My ability to create new applications jumped into hyperspace. It is much more scalable than digital and although you can 'fake' things digitally, they'll always flimsy, depending just on a single byte before they crash. With analogue you have a feature built into the core of the system and than you just polish it with digital. Not the other way around.

Only problem with analogue is that one can forget about doing it intuitively. One really needs to learn maths behind it.

#### DROBNJAK

#14
##### Oct 01, 2013, 08:48 pm

Here's a circuit than sources current (a bit harder than a programmable sink)
http://www.seekic.com/circuit_diagram/Power_Supply_Circuit/VOLTAGE_PROGRAMMABLE_CURRENT_SOURCE.html

I don't understand strange symbol in top right of the picture. It shows +-10V Gnd, but there are no +/-10V rails in the rest of the circuit. Does this circuit require positive and negative voltage power supply?

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