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How to select transistor for oscillator ?

Started by Unknown September 11, 2018
On 09/11/2018 03:29 PM, 698839253X6D445TD@nospam.org wrote:
> John Larkin wrote >> Bipolars are not unstable. But mosfets are usually easier to use. >> >> The last bipolar transistor circuit that I designed was in fact a >> Colpitts oscillator. It's a 600 MHz VCO that uses a coaxial ceramic >> resonator and a BFT25A. The time consuming bit was temperature >> compensating it; I wound up buying a reel of custom-brewed N4700 caps. > > I use bipolars all the time, > Here a 2.4 Giggle Hz oscillator: > http://panteltje.com/pub/2.4GHz_twisted_oscillator_IMG_3629.GIF
The "gimmick" there makes a like 3-5pF cap in that configuration, yeah?
On Tue, 11 Sep 2018 19:29:08 GMT, <698839253X6D445TD@nospam.org>
wrote:

>John Larkin wrote >>Bipolars are not unstable. But mosfets are usually easier to use. >> >>The last bipolar transistor circuit that I designed was in fact a >>Colpitts oscillator. It's a 600 MHz VCO that uses a coaxial ceramic >>resonator and a BFT25A. The time consuming bit was temperature >>compensating it; I wound up buying a reel of custom-brewed N4700 caps. > >I use bipolars all the time, >Here a 2.4 Giggle Hz oscillator: > http://panteltje.com/pub/2.4GHz_twisted_oscillator_IMG_3629.GIF > >1.57 Giggle Hertz: > http://panteltje.com/pub/GPS_jammer_board_twisted_wire_1.57GHz_oscillator_IMG_3622.GIF > >Think those were BFR91. >These are both to some degree frequency controlled by the base current, >that changes Vce and CVce and thus frequency. > >Anyways that was locked to the FPGA board that was locked to the Rubidium reference on the left. > http://panteltje.com/pub/25MHz_lock_test_setup_IMG_3721.GIF > >6 MHz xtal oscillator: > http://panteltje.com/pub/6MHz_xtal_oscillator.gif > >For lower frequencies I of course use unijunction transistors. > >For even higher frequencies there are 'sjips', like this I am working on: > http://panteltje.com/pub/octagon_twin_LNB_OTLSO_inside_RT320M_PLL_IMG_6538.JPG >The RT320M chip is a PLL locked to a 25 MHz xtal (on the right) with an output of >390 x 25 MHz = 9.750 GHz >or >426 x 25 MHz = 10.650 GHz >used as local oscillator and mixer for 10.7 GHz to 12,75 GHz reception. >Modifying it for 10.4 GHz, > >Removed the crystal from the board, here testing if it still works with crystal on thin wires on the side. >Thing is small... bad PCB, tracks fall off... >A tunable external reference will be connected via a connector on the bottom. > http://panteltje.com/pub/octagon_LNB_crystal_removed_IMG_6546.JPG > >Will attempt that locking thing with this LNB too. > >For even higher frequencies I have some various color lasers. > >:-) > > >But anyways bipolars are great, the naming here in Elektor magazine was TUP and TUN for >Transistor Universal NPN and Transistor Universal PNP, >BC547 BC557, also known in a smaller package as >BC847 BC857 >have only a Ft of 100 MHz or so, good enough for most things. > >I also still have an old germanium OC76 and an OC140 germanium transistor in black painted glass envelope. > http://panteltje.com/pub/old_germanium_transistors_IMG_6554.JPG
I have some CK722's! https://www.dropbox.com/s/lq8w4jsh0sn5fc0/Ck722-0A.JPG?raw=1 and some high-voltage opamps https://www.dropbox.com/s/st1upzhqkil8bp6/Philbricks.jpg?raw=1
>
Here's my oscillator: https://www.dropbox.com/s/19imyfg1ubh2z3c/P5_CCRO.jpg?raw=1 The CCROs have TCs, Qs, and initial accuracies over 10x better than an LC. The higher-frequency parts have Qs in the thousands. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
John Larkin wrote
>I have some CK722's! > >https://www.dropbox.com/s/lq8w4jsh0sn5fc0/Ck722-0A.JPG?raw=1
What is the Ft?
>and some high-voltage opamps > >https://www.dropbox.com/s/st1upzhqkil8bp6/Philbricks.jpg?raw=1
hehe, that is funny.
>Here's my oscillator: > >https://www.dropbox.com/s/19imyfg1ubh2z3c/P5_CCRO.jpg?raw=1 > >The CCROs have TCs, Qs, and initial accuracies over 10x better than an >LC. The higher-frequency parts have Qs in the thousands.
Ceramics are not better than quartz crystals I think? The TCXOs are cool, there is one in here: https://www.ebay.com/itm/272411458376 1ppm Using it in my spectrum analyser, good from about 20 MHz to 1.7 GHz http://panteltje.com/pub/xpsa_fm_spectrum.gif and to receive all sorts of radio signals.
bitrex wrote:
>On 09/11/2018 03:29 PM, 698839253X6D445TD@nospam.org wrote: >> I use bipolars all the time, >> Here a 2.4 Giggle Hz oscillator: >> http://panteltje.com/pub/2.4GHz_twisted_oscillator_IMG_3629.GIF > >The "gimmick" there makes a like 3-5pF cap in that configuration, yeah?
I look at it more as a transmission line, or LC cicuit. It will oscillate if serial impedance is low and phase shift close to 180 degrees. Basically it is an LC tuned circuit and phase reversing transformer. . . -||- ) ( ) ( ) ( | | caps all over the length, like a transmission line, inductors coupled 180 degrees phase shift :-) There are probably other ways to look at it, but it works great,
On Tue, 11 Sep 2018 20:57:34 GMT, <698839253X6D445TD@nospam.org>
wrote:

>John Larkin wrote >>I have some CK722's! >> >>https://www.dropbox.com/s/lq8w4jsh0sn5fc0/Ck722-0A.JPG?raw=1 > >What is the Ft? > > >>and some high-voltage opamps >> >>https://www.dropbox.com/s/st1upzhqkil8bp6/Philbricks.jpg?raw=1 > >hehe, that is funny. > > >>Here's my oscillator: >> >>https://www.dropbox.com/s/19imyfg1ubh2z3c/P5_CCRO.jpg?raw=1 >> >>The CCROs have TCs, Qs, and initial accuracies over 10x better than an >>LC. The higher-frequency parts have Qs in the thousands. > >Ceramics are not better than quartz crystals I think?
They are available up into the GHz. As oscillators, they are a lot more pullable than crystals. But most CCROs are probably used as bandpass filters. http://www.mpdigest.com/2017/01/23/ceramic-resonator-band-pass-filter/ -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote:

>On 09/11/2018 11:36 AM, John Larkin wrote: >> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >> >>> Could some electronics guru here please help ? >>> What is the best way to select a transistor >>> for an oscillator ? Consider a Colpitts >>> oscillator running at 5 MHz. The following are >>> the analysis steps. >>> 1. Select a value for the LC tank capacitor >>> C1 = C2 = C >>> 2. Select a load e.g., R=50 Ohm >>> 3. To start oscillations, gmR > C2/C1. In this >>> case gmR > 1 >>> 4. Using the oscillation frequency and the LC >>> tank capacitance value, the inductor value >>> is obtained. >>> 5. Having determined gm, find the collector >>> current using gm = Ic/VT where VT = 0.025 >>> 6. From the VCC value, the collector current >>> limiting resistor is VCC/Ic >>> 7. The base bias resistors are then obtained >>> using the fact that the maximum base bias >>> current is 0.1 times the collector current. >>> Then the question is: how to select a transistor ? >>> >>> All hints/suggestions are helpful. Thanks in advance >> >> >> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >> >> If you care about noise, take a look at Win's table, page 501 of AOE3. >> It pains me that he selected BCX70 as the first/worse transistor in >> the list. >> >> Actually, I rarely use bipolar transistors these days. >> >> > >Unless one really need low phase noise or distortion or very high >frequency or some other specific parameter from an oscillator I think >detailed mathematical analysis of low-frequency oscillators is kinda a >waste of time. Yeah you can do it but the circuits are nonlinear and the >results are often not particularly enlightening. This is what SPICE was >invented for. >
Can Spice do useful noise analysis of oscillators? -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 09/11/2018 10:16 PM, John Larkin wrote:
> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: > >> On 09/11/2018 11:36 AM, John Larkin wrote: >>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>> >>>> Could some electronics guru here please help ? >>>> What is the best way to select a transistor >>>> for an oscillator ? Consider a Colpitts >>>> oscillator running at 5 MHz. The following are >>>> the analysis steps. >>>> 1. Select a value for the LC tank capacitor >>>> C1 = C2 = C >>>> 2. Select a load e.g., R=50 Ohm >>>> 3. To start oscillations, gmR > C2/C1. In this >>>> case gmR > 1 >>>> 4. Using the oscillation frequency and the LC >>>> tank capacitance value, the inductor value >>>> is obtained. >>>> 5. Having determined gm, find the collector >>>> current using gm = Ic/VT where VT = 0.025 >>>> 6. From the VCC value, the collector current >>>> limiting resistor is VCC/Ic >>>> 7. The base bias resistors are then obtained >>>> using the fact that the maximum base bias >>>> current is 0.1 times the collector current. >>>> Then the question is: how to select a transistor ? >>>> >>>> All hints/suggestions are helpful. Thanks in advance >>> >>> >>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>> >>> If you care about noise, take a look at Win's table, page 501 of AOE3. >>> It pains me that he selected BCX70 as the first/worse transistor in >>> the list. >>> >>> Actually, I rarely use bipolar transistors these days. >>> >>> >> >> Unless one really need low phase noise or distortion or very high >> frequency or some other specific parameter from an oscillator I think >> detailed mathematical analysis of low-frequency oscillators is kinda a >> waste of time. Yeah you can do it but the circuits are nonlinear and the >> results are often not particularly enlightening. This is what SPICE was >> invented for. >> > > Can Spice do useful noise analysis of oscillators? > >
Dunno! Can't say I've ever needed to design such a high-performance oscillator myself. If I should need to I have a couple books on my shelf that go thru the math...
On 09/11/2018 10:16 PM, John Larkin wrote:
> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: > >> On 09/11/2018 11:36 AM, John Larkin wrote: >>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>> >>>> Could some electronics guru here please help ? >>>> What is the best way to select a transistor >>>> for an oscillator ? Consider a Colpitts >>>> oscillator running at 5 MHz. The following are >>>> the analysis steps. >>>> 1. Select a value for the LC tank capacitor >>>> C1 = C2 = C >>>> 2. Select a load e.g., R=50 Ohm >>>> 3. To start oscillations, gmR > C2/C1. In this >>>> case gmR > 1 >>>> 4. Using the oscillation frequency and the LC >>>> tank capacitance value, the inductor value >>>> is obtained. >>>> 5. Having determined gm, find the collector >>>> current using gm = Ic/VT where VT = 0.025 >>>> 6. From the VCC value, the collector current >>>> limiting resistor is VCC/Ic >>>> 7. The base bias resistors are then obtained >>>> using the fact that the maximum base bias >>>> current is 0.1 times the collector current. >>>> Then the question is: how to select a transistor ? >>>> >>>> All hints/suggestions are helpful. Thanks in advance >>> >>> >>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>> >>> If you care about noise, take a look at Win's table, page 501 of AOE3. >>> It pains me that he selected BCX70 as the first/worse transistor in >>> the list. >>> >>> Actually, I rarely use bipolar transistors these days. >>> >>> >> >> Unless one really need low phase noise or distortion or very high >> frequency or some other specific parameter from an oscillator I think >> detailed mathematical analysis of low-frequency oscillators is kinda a >> waste of time. Yeah you can do it but the circuits are nonlinear and the >> results are often not particularly enlightening. This is what SPICE was >> invented for. >> > > Can Spice do useful noise analysis of oscillators? > >
The "problem" such as it is, is that OP asked "what the BEST transistor" is and as usual the answer is "it depends." He didn't specify a noise requirement just a 5MHz Colpitts. so why not just a 2N3904? They work fine for the Colpitts at that frequency and there are many examples online. It's kinda a reinvent-the-wheel situation usually when you go to reinvent-the-wheel you a) don't really learn too much and b) end up with a crappy wheel.
On Wednesday, 12 September 2018 04:11:47 UTC+1, bitrex  wrote:
> On 09/11/2018 10:16 PM, John Larkin wrote: > > On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: > > > >> On 09/11/2018 11:36 AM, John Larkin wrote: > >>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: > >>> > >>>> Could some electronics guru here please help ? > >>>> What is the best way to select a transistor > >>>> for an oscillator ? Consider a Colpitts > >>>> oscillator running at 5 MHz. The following are > >>>> the analysis steps. > >>>> 1. Select a value for the LC tank capacitor > >>>> C1 = C2 = C > >>>> 2. Select a load e.g., R=50 Ohm > >>>> 3. To start oscillations, gmR > C2/C1. In this > >>>> case gmR > 1 > >>>> 4. Using the oscillation frequency and the LC > >>>> tank capacitance value, the inductor value > >>>> is obtained. > >>>> 5. Having determined gm, find the collector > >>>> current using gm = Ic/VT where VT = 0.025 > >>>> 6. From the VCC value, the collector current > >>>> limiting resistor is VCC/Ic > >>>> 7. The base bias resistors are then obtained > >>>> using the fact that the maximum base bias > >>>> current is 0.1 times the collector current. > >>>> Then the question is: how to select a transistor ? > >>>> > >>>> All hints/suggestions are helpful. Thanks in advance > >>> > >>> > >>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. > >>> > >>> If you care about noise, take a look at Win's table, page 501 of AOE3. > >>> It pains me that he selected BCX70 as the first/worse transistor in > >>> the list. > >>> > >>> Actually, I rarely use bipolar transistors these days. > >>> > >>> > >> > >> Unless one really need low phase noise or distortion or very high > >> frequency or some other specific parameter from an oscillator I think > >> detailed mathematical analysis of low-frequency oscillators is kinda a > >> waste of time. Yeah you can do it but the circuits are nonlinear and the > >> results are often not particularly enlightening. This is what SPICE was > >> invented for. > >> > > > > Can Spice do useful noise analysis of oscillators? > > > > > > The "problem" such as it is, is that OP asked "what the BEST transistor" > is and as usual the answer is "it depends." He didn't specify a noise > requirement just a 5MHz Colpitts. so why not just a 2N3904? They work > fine for the Colpitts at that frequency and there are many examples > online. > > It's kinda a reinvent-the-wheel situation usually when you go to > reinvent-the-wheel you a) don't really learn too much and b) end up with > a crappy wheel.
Asking what the best thing is is always the wrong approach. But the OP said 'What is the best way to select a transistor for an oscillator ?' NT
On 12/09/18 13:11, bitrex wrote:
> On 09/11/2018 10:16 PM, John Larkin wrote: >> On Tue, 11 Sep 2018 13:32:35 -0400, bitrex <user@example.net> wrote: >> >>> On 09/11/2018 11:36 AM, John Larkin wrote: >>>> On Mon, 10 Sep 2018 20:38:48 -0700 (PDT), dakupoto@gmail.com wrote: >>>> >>>>> Could some electronics guru here please help ? >>>>> What is the best way to select a transistor >>>>> for an oscillator ? Consider a Colpitts >>>>> oscillator running at 5 MHz. The following are >>>>> the analysis steps. >>>>> 1. Select a value for the LC tank capacitor >>>>> &nbsp;&nbsp;&nbsp; C1 = C2 = C >>>>> 2. Select a load e.g., R=50 Ohm >>>>> 3. To start oscillations, gmR > C2/C1. In this >>>>> &nbsp;&nbsp;&nbsp; case gmR > 1 >>>>> 4. Using the oscillation frequency and the LC >>>>> &nbsp;&nbsp;&nbsp; tank capacitance value, the inductor value >>>>> &nbsp;&nbsp;&nbsp; is obtained. >>>>> 5. Having determined gm, find the collector >>>>> &nbsp;&nbsp;&nbsp; current using gm = Ic/VT where VT = 0.025 >>>>> 6. From the VCC value, the collector current >>>>> &nbsp;&nbsp;&nbsp; limiting resistor is VCC/Ic >>>>> 7. The base bias resistors are then obtained >>>>> &nbsp;&nbsp;&nbsp; using the fact that the maximum base bias >>>>> &nbsp;&nbsp;&nbsp; current is 0.1 times the collector current. >>>>> Then the question is: how to select a transistor ? >>>>> >>>>> All hints/suggestions are helpful. Thanks in advance >>>> >>>> >>>> My common NPN gumdrops are BCX70 and BFS17 for faster stuff. >>>> >>>> If you care about noise, take a look at Win's table, page 501 of AOE3. >>>> It pains me that he selected BCX70 as the first/worse transistor in >>>> the list. >>>> >>>> Actually, I rarely use bipolar transistors these days. >>>> >>>> >>> >>> Unless one really need low phase noise or distortion or very high >>> frequency or some other specific parameter from an oscillator I think >>> detailed mathematical analysis of low-frequency oscillators is kinda a >>> waste of time. Yeah you can do it but the circuits are nonlinear and the >>> results are often not particularly enlightening. This is what SPICE was >>> invented for. >>> >> >> Can Spice do useful noise analysis of oscillators? >> >> > > The "problem" such as it is, is that OP asked "what the BEST transistor"
No he didn't. He asked "what is the best way to select a transistor".