On 10/6/2022 1:22 AM, Gerhard Hoffmann wrote:> Am 06.10.22 um 01:51 schrieb amdx: >> Hi all, >> I'm following a group discussing building a Q meter. HP has a two >> transistor Impedance Converter Ass'y. >>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >>> >> One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at >> 1MHz, on up to 7.70Ω at 30MHz after trying several >> common transistors for the first transistor. >> The output transistor is a 2n3866. The first transistor was a >> 2n4401,this gave the lowest output impedance. >> I'm looking for some advice on transistors that will produce the >> lowest output impedance in this circuit. >> Would like to see it lower, especially at he high frequencies. > > The 2N3866 was an overlay transistor as RCA used to call it. > Every emitter stripe had a NiCr resistor in series to balance > the current. That may work against you. > > Cheers, Gerhard > >Thanks for that info, however, I found that the builder actually used the 2N5109. I don't have any 2N5109s, but I do have 2N3866s, so I may need to rethink it's use. Mikek
Picking a transistor for low output impedance circuit
Started by ●October 5, 2022
Reply by ●October 6, 20222022-10-06
Reply by ●October 6, 20222022-10-06
On Wednesday, October 5, 2022 at 7:51:12 PM UTC-4, amdx wrote:> Hi all, > I'm following a group discussing building a Q meter. HP has a two > transistor Impedance Converter Ass'y. > > https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 > One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz, > on up to 7.70Ω at 30MHz after trying several > common transistors for the first transistor. > The output transistor is a 2n3866. The first transistor was a > 2n4401,this gave the lowest output impedance. > I'm looking for some advice on transistors that will produce the > lowest output impedance in this circuit. > Would like to see it lower, especially at he high frequencies. > MikekIs the coupling stepdown a powdered iron? The core loss from 1-30 MHz is possibly very significant and probably responsible for the coupling loss. The frequency dependence is a giveaway. It's only a 1.25 dB deviation- which is not significant for RF.
Reply by ●October 6, 20222022-10-06
On 10/6/2022 10:47 AM, Fred Bloggs wrote:> On Wednesday, October 5, 2022 at 7:51:12 PM UTC-4, amdx wrote: >> Hi all, >> I'm following a group discussing building a Q meter. HP has a two >> transistor Impedance Converter Ass'y. >>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >> One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz, >> on up to 7.70Ω at 30MHz after trying several >> common transistors for the first transistor. >> The output transistor is a 2n3866. The first transistor was a >> 2n4401,this gave the lowest output impedance. >> I'm looking for some advice on transistors that will produce the >> lowest output impedance in this circuit. >> Would like to see it lower, especially at he high frequencies. >> Mikek > Is the coupling stepdown a powdered iron? The core loss from 1-30 MHz is possibly very significant and probably responsible for the coupling loss. The frequency dependence is a giveaway. It's only a 1.25 dB deviation- which is not significant for RF. FT-50-43 50 to 1 transformer. The builder did show a 2 port VNA graph, it's flat, 100kHz to 30MHz, but, I think he was driving a 50Ω on the 1 turn secondary, I don't know what it would look like with 0.5Ω load. Mikek
Reply by ●October 6, 20222022-10-06
On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote:>On 10/5/2022 10:15 PM, John Larkin wrote: >> On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote: >> >>> Hi all, >>> � I'm following a group discussing building a Q meter. HP has a two >>> transistor Impedance Converter Ass'y. >>>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >>> One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz, >>> on up to 7.70? at 30MHz after trying several >>> common transistors for the first transistor. >>> The output transistor is a 2n3866. The first transistor was a >>> 2n4401,this gave the lowest output impedance. >>> �� I'm looking for some advice on transistors that will produce the >>> lowest output impedance in this circuit. >>> �Would like to see it lower, especially at he high frequencies. >>> ��������������������������������������������������� Mikek >> >> A fast opamp would be better. >> >> Or a buffer, like BUF602. It's typically 1.4 ohms at 10 MHz. >> > >If OP is committed to discretes there's always the classic White follower: > ><https://norcim-rc.club/Radio15_files/image025.jpg> > >Which should have substantially better output impedance at high frequency. > >(You can make one of the output transistors PNP and add a third >transistor of either polarity and eliminate the coupling capacitor)A q-meter doesn't need a radically low source impedance. Math it out.
Reply by ●October 6, 20222022-10-06
On 10/6/2022 3:34 PM, John Larkin wrote:> On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote: > >> On 10/5/2022 10:15 PM, John Larkin wrote: >>> On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote: >>> >>>> Hi all, >>>> I'm following a group discussing building a Q meter. HP has a two >>>> transistor Impedance Converter Ass'y. >>>>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >>>> One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz, >>>> on up to 7.70? at 30MHz after trying several >>>> common transistors for the first transistor. >>>> The output transistor is a 2n3866. The first transistor was a >>>> 2n4401,this gave the lowest output impedance. >>>> I'm looking for some advice on transistors that will produce the >>>> lowest output impedance in this circuit. >>>> Would like to see it lower, especially at he high frequencies. >>>> Mikek >>> >>> A fast opamp would be better. >>> >>> Or a buffer, like BUF602. It's typically 1.4 ohms at 10 MHz. >>> >> >> If OP is committed to discretes there's always the classic White follower: >> >> <https://norcim-rc.club/Radio15_files/image025.jpg> >> >> Which should have substantially better output impedance at high frequency. >> >> (You can make one of the output transistors PNP and add a third >> transistor of either polarity and eliminate the coupling capacitor) > > A q-meter doesn't need a radically low source impedance. Math it out. >I meant better than the Darlington, I'd be surprised if two transistors could beat a BUF602 at 10MHz. But I don't know what's in the BUF the datasheet doesn't seem to be telling. 8000 V/us seems ludicrously fast, though
Reply by ●October 7, 20222022-10-07
On 10/6/2022 2:34 PM, John Larkin wrote:> On Wed, 5 Oct 2022 22:38:02 -0400, bitrex <user@example.net> wrote: > >> On 10/5/2022 10:15 PM, John Larkin wrote: >>> On Wed, 5 Oct 2022 18:51:04 -0500, amdx <amdx@knology.net> wrote: >>> >>>> Hi all, >>>> I'm following a group discussing building a Q meter. HP has a two >>>> transistor Impedance Converter Ass'y. >>>>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >>>> One builder got down to 0.86? output impedance at 50kHz, 1.05? at 1MHz, >>>> on up to 7.70? at 30MHz after trying several >>>> common transistors for the first transistor. >>>> The output transistor is a 2n3866. The first transistor was a >>>> 2n4401,this gave the lowest output impedance. >>>> I'm looking for some advice on transistors that will produce the >>>> lowest output impedance in this circuit. >>>> Would like to see it lower, especially at he high frequencies. >>>> Mikek >>> A fast opamp would be better. >>> >>> Or a buffer, like BUF602. It's typically 1.4 ohms at 10 MHz. >>> >> If OP is committed to discretes there's always the classic White follower: >> >> <https://norcim-rc.club/Radio15_files/image025.jpg> >> >> Which should have substantially better output impedance at high frequency. >> >> (You can make one of the output transistors PNP and add a third >> transistor of either polarity and eliminate the coupling capacitor) > A q-meter doesn't need a radically low source impedance. Math it out. > John that's true, but in a system where all you can know or measure is the output of your signal generator, a higher source impedance does introduce more error. I probably need something clarified, on the HP4342A, they put a 75Ω resistor across the transformer primary, The design under discussion uses a 50Ω. What is the purpose of that resistor? With a 50Ω system, the error comes from the voltage divider 50Ω source driving a parallel 50Ω//750Ω (=46.88Ω), where 750 is the secondary load (0.3Ω) reflected back to the primary (50 to 1 turns ratio). Vout = 2V x 46.88 / (50 + 46.88) = 46.88 / 96.88 = 0.968V for a 3.2% error. With a 2Ω source resistance and 600Ω across the transformer, the error is 0.6%, but then if the source resistance rises to 5Ω,the error grows to over 9%. So if you have low source resistance, a constant value of source resistance is important, otherwise, your better off with a 50Ω system. Mikek
Reply by ●October 7, 20222022-10-07
On 06/10/2022 00:51, amdx wrote:> Hi all, > I'm following a group discussing building a Q meter. HP has a two > transistor Impedance Converter Ass'y. >> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >> > One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at 1MHz, > on up to 7.70Ω at 30MHz after trying several > common transistors for the first transistor. > The output transistor is a 2n3866. The first transistor was a > 2n4401,this gave the lowest output impedance. > I'm looking for some advice on transistors that will produce the > lowest output impedance in this circuit. > Would like to see it lower, especially at he high frequencies. > Mikek >I quite like npn-pnp-npn triples for extreme followers. See fig 16 in the ever great Rod Elliott page on followers: <https://sound-au.com/articles/followers.html> piglet
Reply by ●October 7, 20222022-10-07
piglet wrote:> On 06/10/2022 00:51, amdx wrote: >> Hi all, >> I'm following a group discussing building a Q meter. HP has a two >> transistor Impedance Converter Ass'y. >>> https://www.dropbox.com/s/n5gj8powzph6611/HP4342A%20impedance%20converter%20and%20injection%20transformer..jpg?dl=0 >>> >> One builder got down to 0.86Ω output impedance at 50kHz, 1.05Ω at >> 1MHz, on up to 7.70Ω at 30MHz after trying several >> common transistors for the first transistor. >> The output transistor is a 2n3866. The first transistor was a >> 2n4401,this gave the lowest output impedance. >> I'm looking for some advice on transistors that will produce the >> lowest output impedance in this circuit. >> Would like to see it lower, especially at he high frequencies. >> Mikek >> > > I quite like npn-pnp-npn triples for extreme followers. See fig 16 in > the ever great Rod Elliott page on followers: > > <https://sound-au.com/articles/followers.html> > > piglet >You can make that work if you take care that it doesn't oscillate. Followers driving followers have a bad rep for that, unless (a) the whole thing is running at sufficiently low current, or (b) there's a base stopper on the output stage. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
