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=0One 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
Picking a transistor for low output impedance circuit
Started by ●October 5, 2022
Reply by ●October 5, 20222022-10-05
On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, 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.Well, it's a Darlington; the impedance goes down when you either use a higher bias current, or replace a transistor with a germanium one... Since this is AC-coupled, can you use a stepdown transformer to do the coupling? That will get the impedance lower... so would some positive feedback.
Reply by ●October 5, 20222022-10-05
On 10/5/2022 8:24 PM, whit3rd wrote:> On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, 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. > Well, it's a Darlington; the impedance goes down when you > either use a higher bias current, or replace a transistor with a > germanium one... > > Since this is AC-coupled, can you use a stepdown transformer > to do the coupling? That will get the impedance lower... so > would some positive feedback. Do you mean on the input? Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω? Mikek
Reply by ●October 5, 20222022-10-05
On Wednesday, October 5, 2022 at 9:43:03 PM UTC-4, amdx wrote:> On 10/5/2022 8:24 PM, whit3rd wrote: > > On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, 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. > > Well, it's a Darlington; the impedance goes down when you > > either use a higher bias current, or replace a transistor with a > > germanium one... > > > > Since this is AC-coupled, can you use a stepdown transformer > > to do the coupling? That will get the impedance lower... so > > would some positive feedback. > Do you mean on the input? > Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω?The output impedance of your circuit is controlled by the current gain of the two transistors and the resistors R4 and R5. Use transistors with higher current gain and/or adjust the resistor values to put the transistors on higher current gain portions of their curves. More current gain provides more current capability to reduce the output impedance. As was already asked, the transformer actually sets the output impedance from the amplifier stage. Why not change that to give lower output impedance? Lower impedance, but lower voltage. Do you know the turns ratio? Is it 1:1? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
Reply by ●October 5, 20222022-10-05
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. > ��������������������������������������������������� MikekA fast opamp would be better. Or a buffer, like BUF602. It's typically 1.4 ohms at 10 MHz.
Reply by ●October 5, 20222022-10-05
On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, 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.> Would like to see it lower, especially at the high frequencies.Another thought: PNP output transistor (NPN/PNP pseudodarlington circuit variant) might help; the base spreading resistance in the output stage is higher for P-doped base than it is for N-doped. 2N3866 is in TO-39 case? That's OLD silicon. How about 2SA2124 or CZT955? The emitter output impedance is the low-Z target, I'd guess; for positive feedback, you can run emitter current through a bead with a secondary winding to boost the RF base current... but gently. I'd want to SPICE it before committing that to a power-up test.
Reply by ●October 5, 20222022-10-05
On 10/5/2022 9:00 PM, Ricky wrote:> On Wednesday, October 5, 2022 at 9:43:03 PM UTC-4, amdx wrote: >> On 10/5/2022 8:24 PM, whit3rd wrote: >>> On Wednesday, October 5, 2022 at 4:51:12 PM UTC-7, 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. >>> Well, it's a Darlington; the impedance goes down when you >>> either use a higher bias current, or replace a transistor with a >>> germanium one... >>> >>> Since this is AC-coupled, can you use a stepdown transformer >>> to do the coupling? That will get the impedance lower... so >>> would some positive feedback. >> Do you mean on the input? >> Maybe a 50Ω to 12.5Ω, and change the 56Ω resistor to 12Ω? > The output impedance of your circuit is controlled by the current gain of the two transistors and the resistors R4 and R5. Use transistors with higher current gain and/or adjust the resistor values to put the transistors on higher current gain portions of their curves. More current gain provides more current capability to reduce the output impedance. > > As was already asked, the transformer actually sets the output impedance from the amplifier stage. Why not change that to give lower output impedance? Lower impedance, but lower voltage. Do you know the turns ratio? Is it 1:1? > I was hoping someone would have a part number for a transistor higher current gain than a 2n4401 and/or a different output transistor. The transformer is 50 to 1, 2500 impedance ratio. The one turn secondary sees roughly 0.3Ω to 3Ω. The circuit draws 117 mA; the output transistor draws 84 mA. Driving with a 50Ω signal generator, the change of the R on the secondary, reflected back to the primary caused a 2% reduction in the drive voltage, so it was thought the HP impedance converter will correct a lot of that error. Mikek
Reply by ●October 5, 20222022-10-05
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)
Reply by ●October 6, 20222022-10-06
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 >In the usual technician-course model, The output impedance of a BJT follower at lowish frequency is basically the sum of two terms: first, the small-signal emitter resistance (25 ohms/I_C in mA), and second, the impedance seen by the base divided by the AC beta. Both stages run pretty hot, with Q1's I_C ~ 20 mA and Q2's around 85 mA give or take. Q1 therefore has a Zout of a bit less than 2 ohms, based on an AC beta of 40 or 50 and the 25-ohm impedance at the base of Q1. (I assume that J6 normally connects to a 50-ohm source, but it doesn't make much difference.) So the source impedance seen by Q2 is very small, so that contribution to its Zout is small squared. The output impedance in the simple model is thus 25 ohms / 85 mA, about 300 milliohms. In real life, besides AC effects, there are extrinsic resistances in the base and emitter of Q2 to consider, which are usually on the order of an ohm in medium-power devices. Q2 has a quiescent dissipation of around (25V*562/2612 + 0.7)**2 / 220 = 170 mW, so you're probably looking at a TO-5 can device. BITD my go-to for that sort of job was the 2N5109. A 2N4401 is reasonable, but its f_T is only about 300 MHz. From things in my junkbox, I'd probably start with a BFU520A or BFS17 and rebias it a bit to reduce the dissipation, maybe with a parallel RC in its collector. 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
Reply by ●October 6, 20222022-10-06
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
