Let's made a PC-base metal detector with usb interface !!!

[Aziz]

Dual Matched Transistor Pairs

Hi all,

after playing around with test circuit for Vbe (Base-Emitter voltage drop) and hFE (Beta or DC current gain), I came to the conclusion, that it isn't worth trying to find matched transistor pairs.

The temperature gradient is a big issue. Both transistors needs to be placed at same substrate (chip die). For matching to both parameters, the number of necessary transistors would be huge (200 - 500). So the found matched pair could still not achieve the performance of a MAT02 for instance. It's still a big problem to measure the parameters at slightly varying room temperature (air convection, wind blow, handling of the transistor, measurement accuracy of my DMM, ...).

I should either invest all my effort into getting these super matched transistor pairs and/or the VCA chip instead.

BTW, my last parts order is not arrived yet.


Aziz

[Qiaozhi]

Hi all,

after playing around with test circuit for Vbe (Base-Emitter voltage drop) and hFE (Beta or DC current gain), I came to the conclusion, that it isn't worth trying to find matched transistor pairs.

The temperature gradient is a big issue. Both transistors needs to be placed at same substrate (chip die). For matching to both parameters, the number of necessary transistors would be huge (200 - 500). So the found matched pair could still not achieve the performance of a MAT02 for instance. It's still a big problem to measure the parameters at slightly varying room temperature (air convection, wind blow, handling of the transistor, measurement accuracy of my DMM, ...).

I should either invest all my effort into getting these super matched transistor pairs and/or the VCA chip instead.

BTW, my last parts order is not arrived yet.


Aziz

There are several other OTAs available here ->
http://uk.farnell.com/jsp/search/bro...atchallpartial

[Aziz]

Hi Qiaozhi,

There are several other OTAs available here ->
http://uk.farnell.com/jsp/search/browse.jsp?N=500006+1004434&Ntk=gensearch_001&Ntt= transconductance&Ntx=mode+matchallpartial

Thanks for your effort. I did order some of the LM13700 already but they did not arrive yet.

OTA as a VCA is not my preferred solution. Either an integrated VCA-chip or a discrete differential stage with programmable bias current (using some of the MATxx/SSM22x0 dinosaurs). The first one would reduce the parts count (VCA810 would be very fine). The latter one would have best noise performance.

BTW, I have finished my first hand matched BC550C transistor pair:
Vbe offset voltage match: <1 mV (approx. 700 µV)
hFE match: 0.2% (1 of 608 )
Found in a batch of only 50 BC550C NPN transistors.

It is already tested in a differential amplifier stage: It adds only 10 dB noise (gain x100, grounded inputs).

Well the temperature gradient is not much critical, as we can compensate this with multiple sampling windows (subtracting sampling windows).

I will use the hand matched transistors as a replacement for the better ones until I can get them.

I will make more hand matched samples soon.

Aziz

[Aziz]

New Parts Arrived Finally

Hi all,

Ok, I have got the new parts finally. They came today.

IB-PI project:
I can continue with the IB-PI project now (optimized for high coil currents).
There are several possible ways for the processing:
- frequency domain (as usual on the harmonic fundamentals of the TX frequency)
- time domain (digital integration over sampling time windows)

The time domain analysis is not done yet and I am looking forward to see some results. This requires a software synchronisation due to operating system/sound-card latency times.


PI Controller project:
Still collecting parts. In the mean time, I can get the other parts for the PI controller project. I am particularly looking after the super-matched transistor pairs, VCA chip, MUX-chips, fet's. Also a comparison of the hand matched to the super-matched transistor pairs would be very interesting to see.

Well, it's going forward at a snail's pace. But it's going forward.

Aziz

[Aziz]

Off-topic Serious Concern

Hi friends,

it's time again for criminal assaults by the incredible perverted and degenerated german justice. So I have to act as an individual lawyer to act against unlawful demands by the german government and justice. Yet again.

As the criminals do not understand yet, that crime does not pay (still valid on everywhere on the planet), I am willing to accept illegal arrestment by the german government for a duration of max. six month. This will cause at least 100 EUR / day cost for them, so that crime really will not pay off.


Costs: 180 days * 100 EUR = 18,000 EUR (minimum of course, but I will give my best to increase it radically)


I can not imagine, that the fool criminals will accept this crazy trade. But I count on their totally ignorance and criminal intent.

I am sorry again, but it will definitely delay the current projects. So be aware of it.

Peace,

Aziz

[Aziz]

Matching Transistors for Differential-Pair

Hi all,

I have started again to work on transistor matching. My DMM is not accurate enough (only 4000 counts). But hell, why not using the sound card for much more dynamic range (six decades!)?

I will work on a test board, which will make it elegantly. And this on a test board destined for differential pair with the intended operating collector currents. It can be used to match resistor pairs too. Of course, noise behaviour can also be measured. So I can try different transistors for low noise operation.

The tricky solution: CMRR analysis (with shorted differential inputs to ground). More on later.

The thermal coupling of the transistors must be done through the leads of the transistors. The plastic case is a bad heat conductor. But the collector lead is going directly to the chip die. The base and emitter leads are not good enough (wired with a very thin gold wire to the chip die). Anyway, they conduct the heat to the plastic case good.


Aziz

[Aziz]

Surprising Low Noise Transistor

Hi all,

I have built the mentioned test board (differential pair). It seems to be a very fine transistor matching system (hFE and Ube) for NPN and PNP transistors (just exchanging the power supply polarity). I can get 4-5 digits matching accuracy with the sound card measurement using the 24-bit resolution. More on later.

But I have found, that the BC327-40 (PNP)/BC337-40 (NPN) transistors work more quiet than the BC550C (NPN)/BC560C (PNP) variants. 3 dB lower noise figure (BC327-40 tested)! This is quite surprising, because there are no noise figure (NF) specifications for them yet.

My measurements confirm the benefit of BC327-40/BC337-40 mentioned in the following paper:
http://www.janascard.cz/PDF/Ultra%20...amplifiers.pdf

Really funny thing. I will buy some super-matched transistor pairs next month to compare them to the hand-matched types.

Aziz

[WM6]

But I have found, that the BC327-40 (PNP)/BC337-40 (NPN) transistors work more quiet than the BC550C (NPN)/BC560C (PNP) variants. 3 dB lower noise figure (BC327-40 tested)! This is quite surprising, because there are no noise figure (NF) specifications for them yet.

Thanks for interesting info Aziz. Enjoy your work.

[Aziz]

BC337-40 (NPN)

Hi all,

the BC337-40 (NPN) is also tested now. It achieves very good noise performance as well (at collector current of 1.2 mA).

Both outputs of the differential pair stage were fed into the sound card input line (left and right channel). The difference is made digitally of course. My noise floor level is at -114 dB (no input plug connected to the sound card). When I plug the test board, the noise floor increases only by 1 dB into -113 dB.
There is a signal level distance of 4-5 dB to the BC550C pair (noise floor at -108 dB).

So the BC337-40 and BC327-40 are working outstanding. I will order some more samples of these transistors soon (I had approx. 15 pieces of each already and found a good match in the batch).

Aziz

[Aziz]

Differential-Pair Transistor Matcher

Hi all,

here is the differential-pair transistor matcher. I have found this proposal somewhere in the internet forums.

First try to find two same 2.2k collector resistors (measure this with your DMM). Their matching accuracy should be better than 1%. Otherwise the offset voltage might be too much and the operating point moves into the saturation region of the transistors. If you have very accurate collector resistors, R9,R10 and U1 can be dropped.

Select the collector current (no signal fed into the inp). The pot U2 is for adjusting this. You can measure the collector current if you measure the voltage drop across R1 or R2.
Collector current = UdropR1/R1 = UdropR1/2200
For most applications, a collector current in the range of 1 mA is ok. So a voltage of 2.2V across the resistor R1 should be there.

Feed an AC signal into the inp input (1 kHz sine wave, Sound card/function generator). If both transistors Q1 and Q2 are well matched, the AC and DC voltage across the transistor collector will be low. The lower, the better matched. The DC voltage is the amplified offset voltage mismatch (Vbe mismatch). The offset voltage gain is around x80 I think (spice simulation).

Matching strategies:
1. You can compare REF against DUT (minimising the measured DC/AC voltages).

2. Or you can change the DUT's and note the measured DC/AC voltages. The two almost identical measured DUT's do match together.

Both transistors need a thermo coupling. If the temperature is differing, DC and AC voltages will heavily change. I recommend to place the whole circuit in a box during measurement. After both transistors get same temperature, the measurement can be done.

You can use any DMM for DC/AC voltage measurements. Or you can use the sound card.

Aziz

[Aziz]

Next Part's Order

Hi all,

I have just ordered another bunch of interesting parts. This time two super-matched transistor pairs, which will (hopefully) be available:
LM394 (NPN)
MAT03 (PNP)
(no MAT02's there... )


I don't know, whether the hand-matched transistor pairs will be better (quiet). So I have ordered a batch of:
100x BC337-40
100x BC327-40

100x 2SA1015
100x 2SC1815
Let's look, how the Japanese transistors work. They claim to have a typical noise figure of 1 dB. Can they outperform the BC337/BC327? And how the super-matched will work? Well, I will see this soon.

At least, I will have a life-time stock of these transistors above.

Aziz

[WM6]

At least, I will have a life-time stock of these transistors above.

Aziz

Hi Aziz,
where you decided to order?

[Aziz]

Hi Aziz,
where you decided to order?

here:

http://www.reichelt.de

Aziz

[Aziz]

Temperature-Controlled Differential Pair

Hi all,

look at this obsolete part: µA726
(see below data sheet)

I even have been thinking of doing almost the same. But in a different and state of the art variant.

Aziz

[Aziz]

New Part's Arrived

Hi all,

the new parts just arrived and I am very happy to have them.
Now the very interesting questions are awaiting to be answered.

It's really very difficult to match the transistors due to ambient temperature changes. I probably will build a temperature controller to keep them at specified temperature. I also have ordered some temperature sensors (LM335) for this purpose and for the PI Controller.

A lot of important parts in the PI Controller are temperature sensitive. I will track them and process the tempco in the software.

Let's go..

Aziz