~ Transistor Noise and rbb measurement ~

What prompted this web page was that I had used specific audio low noise transistors for microphone and moving coil RIAA amplifiers from 1980 and recently saw a comment that the Hitachi 2SA1085E and 2SC2574E transistors I often used had a much higher base spreading resistance [rbb] than my noise measurements on actual amplifiers suggested

Possibly the best reference on transistor noise was from Dr. E. A. Faulkner in 1968 following the introduction of the silicon planar transistor manufacturing technique around 1959 [it was a good year] which was originally developed to make better integrated circuits but in doing so Fairchild [now Onsemi] also made much quieter single transistors

Throughout the 1970s transistors improved but this levelled out around 1980 with regard to low noise single transistors for audio and video amplification ~ New low noise microwave devices appeared but transistors for audio amplifiers were so good that specific devices were only needed for very low level signals from low source impedances

As explained in the references below any resistance in series with the input to an amplifier including the resistance of the source will produce thermal noise that gets amplified and may be comparable to the output signal ~ When the source is say a ribbon microphone or a moving coil cartridge all additional resistances around the input need to be very low

There is an internal resistor r_e in bipolar transistors in series with the emitter which limits the maximum transconductance gain [voltage in controls current out] by providing local feedback in series with the input but this resistor which varies with collector current and temperature does not produce any thermal noise voltage ~ Only 'metallic' resistors like rbb do

The connections to a transistor chip and any silicon [or semiconductor material] that is not involved with transistor action will have metallic resistance ~ The collector and emitter resistances are low but depending on the design the material in the narrow base region that is not providing transistor action appears as a metallic resistor rbb or r_bb'

I will use rbb rather than the correct r_bb' to indicate that this resistor is internal to the transistor and is placed between the base connection and the semiconductor base material that is performing the transistor action ~ older articles may use rb and in USA rx ~ because of its nature rbb is difficult to isolate to measure and as such is not on many data sheets

Ribbon microphones and some moving coil cartridges have impedances of 10s of ohms or less and signal outputs less than 1mV ~ The traditional 'low noise' transistors like the BC109 have base series resistance rbb around 400Ω Ref.1 p32 so a single device used to amplify a 1mV signal would have S/N about 23dB [NF] worse than the source Signal/Noise

DIY–Audio had an article about an interesting and somewhat convoluted rbb measuring device called the Black Knight but its calculated rbb values for ZTX851 BC337 etc. were way off the mark compared to those measured by Paul Horowitz and Winfield Hill [ref. 2] and myself ~ It also showed a schematic from Dick Kleijer without credit

Other articles and some often quoted books on the subject use mathematics to calculate rbb from manufacturers data sheet noise curves or single point input noise voltages but these figures are conservative and in the past [when low noise transistor rbb was ≈400Ω] were made with test gear that had 10s of Ω in the transistor input path

The topology of the circuit shown is often used as a low noise moving coil [MC] head amplifier but others are available ~ The McCormick [BV] method does not appear to take into account shot and thermal noise across the DUT load and the op–amp is used as a x1 and x0.708 [0.7071 ?] voltage amplifier with ≈50kΩ noise from R5//R6 at its input

Using an op–amp requires a good one and possibly 2 jigs for NPN and PNP transistors but with the transistor under test in the feedback loop and the op–amp configured as a transimpedance amplifier [TIA where current input controls voltage output] the gain of the transistor in common emitter mode is not important but the overall gain is

Comparing my schematic with that of DK I have R5 as a current source from an additional –ve supply and the base of DUT is connected to ground 0V via R4 which eliminates the need for some large value capacitors and a variable bias supply ~ The circuit was powered from Nimh batteries and appeared to give meaningful results but:

I did not use an additional 60dB amplifier as DK had done but used a HP35670A DSA to supply the input signal [Vs] and directly measured the output noise in 1Hz and other bandwidths ~ Using the DK calculator the rbb value results appeared reasonable until I checked some transistors and added 10Ω and 100Ω wire wound resistors in the base lead

Adding 10Ω in the DUT base circuit should make the calculated rrb higher by 10Ω and similar for other values of additional R but this was not the case ~ The DK calculator did not appear to take into consideration that base current shot noise flows through rbb and the 1Ω resistor and that collector current shot noise also flows through the 1Ω resistor

If the input of the op-amp were a perfect TIA [zero Ω] which contributed no noise then only the collector and base current shot noises through r_e would need to be subtracted from the output noise to calculate rbb ~ but base shot current also flowing through the input circuit resistors leads to the solution for rbb being a quadratic equation

My rbb from data sheet calculator already used the solution of a quadratic to obtain rbb from total collector voltage noise ~ but applying it to noise measured with the jig still did not give consistent results when additional known resistors were added in series with rbb ~ probably due to DUT being part of the TIA feedback or some unaccounted for noise

 

A simple r_bb' test jig for connection to an Audio Spectrum Analyser or DSA

 

A concern I had reading about and trying the other methods was ensuring the capacitors had very low impedance ~ I started with a few 10µF ceramics in parallel with a supercap but the results were not as expected ~ Even 2 or 3 0.3F in parallel gave errors of several Ω ?  ~ The data sheet for these supercaps showed ESR in mΩ and it was 4800mΩ !

I am not a great fan of bipolar capacitors but I had used these for repair and modification of QUAD 405s in the past and they work well and hold their characteristic to 100s of kHz and in parallel with 3x 10µF very low ESR ceramic capacitors the rbb noise spectrum was flat between 20Hz and 100kHz if required and ESR does not produce noise

Several ways of powering the jig were tried including a bank of Maxwell supercaps which unlike the small supercaps that did not work as an emitter—base shunt have ESR ≈ 3mΩ each so the 8 in series for the 20V collector supply have an impedance of about 24mΩ which I decided not to include with R1 but a floating power supply is required

The switches are 4 gang gold contact signal type with all gangs paralleled which were soldered with the contacts open to prevent heat transfer damage ~ Measurements can be made with R4 in circuit and 1Ω subtracted from the calculation but when the base is switched to the ground plane accurate low rbb can be measured and R4 then switched in to confirm an increase of 1Ω

Fake low noise 2SA1085E with high r_bb'

In the picture above I am testing some 2SA1085E that turned out to be Fake with rbb of more than 100Ω ~ I have not checked if they have a 120V collector breakdown but very low noise they are not ~ They were purchased from Nikko UK over 5 years ago and are still being listed by ebay sellers in 2024 ~ The batch code is 9A 2 and there is a picture of the fake devices here ~ Also I would avoid GE 1 and JX K and any devices that should be over 30 years old but have shiny legs ~ Contact me if you have other suspect devices

Sample measurement of r_bb' and r_o with simple test jig and calculator

V_RC measured across R1 was 14.7V so the collector current I_C is 2.185mA and the noise in the 35670A calculated band of 10kHz [2kHz to 12kHz] using the Hann window is 21.24µV so rbb is 4.7972Ω lets call it 4.8Ω and r_o is a surprisingly low 55kΩ

The ambient temperature was about 22˚C and with I_C = 2.185mA it was estimated that the junction was at about 25˚C assuming [!] the thermal resistance of the T092 body is 250˚C/W and there is ≈6V across the transistor ~ if I_C = 10mA then the junction would be at ≈37˚C and if this was not accounted for the calculated rbb would be ≈16% lower

The current gain  β or hfe of the DUT is also required but can often be taken from the data sheet or measured by switching the 1kΩ resistor R5 in and measuring the voltage across it to get the base current ~ collector current is measured by voltage across R1 and  β= I_C/I_B or with this jig with R1 = 6727Ω is 6.72 x V_R1/V_R5 

For more information about transistor noise and low noise amplifiers see:

Ref.1  The design of Low–noise audio frequency amplifiers ~ By E. A. Faulkner ~ The R & E E July 1968
Ref.2 The Art of Electronics 3rd edition chapter 8 ~ By Paul Horowitz and Winfield Hill
Ref.3  Noise in Transistor Circuits ~ By P. J. Baxandall ~ Wireless World November 1968
Ref.4  Designing low–noise audio amplifiers ~ By Wilfried Adam ~ Wireless World June 1989
Ref.5  Introduction to low–noise amplifier design ~ By A. Foord ~ Wireless World April 1981

" One of these ~ Surely will screen out the sorrow "