VCO: Negative resistance VCO

2014/7/19

negative resistance device is novel and powerful, most of Negative resistance VFO/VCO is ultra wide bandwidth, easy starting oscillation.

Actually negative resistor oscillator is a method of understanding the circuit, not the circuit name. it's identical, loop Gain>1 identical to gm>1/Rp. (some negative resistance requirement > 2*/Rp)  

one of well known BJT negative resistor is one BJT leave the base fly, novel led flasher:

and there is many many interesting circuits like this:

http://members.shaw.ca/roma/twenty-three.html

popular configuration of such negative resistance is 2 NPN BJT connect as a long tail, i,e: VCO: Ultra Negtive 100kHz-100Mhz.  sometime it's called push pull, or differential pair, it's actually identical. 

this type of configuration had many many different configuration, as AC point of view, they are essentially same:

Lambda diode is typical negative resistance device. i never had one lambda Diode, fortunately, there are several configuration could be use as Lambda style 2 port negative device.

the 2N3820 is unlikely in the junk box, but right photo is just what you want.  negative active device is really fancy,  Oscillator, VK5BR's Dip, Regen.  just because it's really easy to go.

Note about this lambda device:

*100k 20k could replace with 33k/6.8k for high frequency, that need more gain and current.

* at VHF, like 100Mhz,  parallel a 100pF capacitor ( experimental )  with 100k resistor.  i think this capacitor must be add. refer to 2N3820, which also AC shorted.

i first time see this style oscillator is trying to get a wide range VCO, VCO: Super wide band linearity VCO, which come from END's article, cle  Series LC tank VCO breaks tuning range records.

this UJT configuration very similer with the long tail version, but use PNP+NPN, and different output/input.

Other type negative resistance Oscillator

* this is a series tank VCO, include the Q1 be Junk as part of tank, as negative resistance.

* the cascode configuration reduce Miller effect. 

* output is well decoupled

Another VCO actually not negative resistance style, it internal capacitance, Cbe, Cbc form a Clap oscillator.

flow the rule  " connect one transistor's base to another's collector",   change the long tail configuration to one PNP one NPN, we get two more negative resistor.  there are actually a  Common Collector Amplifier cascade with a 'semi' common base PNP buffer(which base not totally been bypassed).  different to the long tail, this is a real TWO PORT device, then easy to deploy as a negative resistor.

Fist new configuration all BJT negistor:

* 2 couple capacitor 3pF to 330pF, choose based on frequency, generally, few pf is enough.

* i setup up a type B oscillator to testing this negistor, use 2N2222/2N3906  it easy start oscillation at 100Mhz.

* type B oscillator had around 2:1 tune ratio.

Selenium from theradioboard post a comments utilize this little circuit as a regen receiver, and get DC bias a little bit simple.  

Second new configuration all BJT negistor:

before construct the real HW, this sch could be simplify :

you can not directly connect this to a tank there need a resistor setup the proper bias:

Let's check the working HW: 

@ 2V and few mA, oscillator work well, when increase voltage, waveform will severely distortion, you can increase 330R resistor to get perfect sine. (by eye)

this oscillator had a very stable amplitude vs frequncy response, detailed information refer to: VFO:New negative 'zoom' oscillator

Reference:

http://litetec.hubpages.com/hub/Negative-Dynamic-Resistance-Circuit

http://www.zen22142.zen.co.uk/Theory/neg_resistance/Oscillations%20and%20Regenerative%20Amplification%20using%20Negative%20Resistance%20Devices.pdf

http://users.tpg.com.au/users/ldbutler/NegResDipMeter.htm

http://www.epanorama.net/sff/Components/The%20Lambda%20Diode%20-%20A%20negative%20resistance%20device.pdf

http://electronbunker.ca/NegativeResistance.html

http://www.qsl.net/va3iul/High_Frequency_VCO_Design_and_Schematics/High_Frequency_VCO_Design_and_Schematics.htm

UJT:

http://www.rfcafe.com/references/popular-electronics/getting-know-ujt-april-1970-popular-electronics.htm

http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

http://theradioboard.com/rb/viewtopic.php?f=4&t=5332&start=15

http://electronics.stackexchange.com/questions/28864/how-to-build-a-2n6027-put