04-03-2017, 09:02 PM
Thanks for the extra info on the cartridge, Mike.
The obvious thing that leaps out is the recommended load impedance of 47k. Now, that's more like what I'd expect for a magnetic MM cartridge.
In light of earlier comments, and to help me relax after battling the fence, let's reach for the breadboard!
The first problem I had was finding a BC109 with a hfe of 200 (I was hoping to match practice with our earlier calculations). Most of mine are 400 or more, but I found one that was almost exactly 300. OK, so we'll have to run the numbers again.
Let's recalculate the input impedance: without the 5M6, we've got 300 times (re + 2k2). re is still 45 ohms as gm hasn't changed. So that gives us 675k. Quite respectable.
What about the Miller effect on the the resistor? Well, assuming the voltage gain is x8.2, then it'll look like 610k. So...
Miller effect is more than halving the input impedance to 320k, so I'd say that's quite an affect. Nothing like what I erroneously suggested initially, but plenty. On paper, at least. The 2n2 cap changes all that. More on that story later...
On breadboard, fed from a 50 ohm source, and with no load beyond the 1M of the 'scope, the built circuit has a voltage gain of 7.64. As expected, it's not as high as the 8.2 suggested by the simple RL/RE approximation.
Next, I measured Zin using the added resistance method, using an old RS decade box in series with the input, and adjusting for half the output amplitude. I got 310k. Not bad!
For fun, I decided to eliminate Miller effect. To do that, I simply split the 5M6 in half, and grounded the mid-point via a capacitor. This obviously maintains the DC conditions, and the AC gain didn't change because of the relatively low open-loop gain.
Now Zin is 500k. Not as high as 675k, but to be honest, I wasn't surprised at that result, given that we're using simplified models and grotty breadboards. Also note that the 5M6 is being "Millered" slightly less in reality (because of the reduced gain) to 650k. But happily these "practical realities" converge nicely to give an overall input impedance that is within 10k (3%) of the prediction. That's pretty good, really...
Yes, I initially saw 2k2 and 2n2 and thought "33kHz" and ignored it. But 5M6 and 2n2 screams much louder! We can't ignore it after all.
I added the 2k2 and 2n2 cap. I've put the single 5M6 back in, so the circuit looks exactly like the original.
Fed from 50 ohms, nothing much changes at 1kHz. The -3dB point of the circuit has reduced (not surprisingly!) from ~70kHz to 4.3kHz.
But obviously the cartridge won't have a 50 ohm source impedance. That isn't specified for this model. So all we can sensibly do is plot Zin against frequency:
Quite a range!
For the record (groan!), I did check Zin vs frequency without the 2n2/2k2 network, and found that it falls to half of 310k at about 20kHz. Much less dramatic.
So for a given source impedance, the frequency response will change considerably as a result of this.
What does the 2k2 resistor do? I found that shorting it added overshoot to a square wave response. That's good enough for me - I didn't explore further.
So there we are. An apparently simple circuit that hides a wealth of intrigue within its many layers. And like the proverbial onion, the more you attempt to peel them back, the more you want to cry!
So when poor Mike innocently asked the simple question about input impedance, who knew it would be this involved?
Hope this is of interest!
Mark
The obvious thing that leaps out is the recommended load impedance of 47k. Now, that's more like what I'd expect for a magnetic MM cartridge.
In light of earlier comments, and to help me relax after battling the fence, let's reach for the breadboard!
The first problem I had was finding a BC109 with a hfe of 200 (I was hoping to match practice with our earlier calculations). Most of mine are 400 or more, but I found one that was almost exactly 300. OK, so we'll have to run the numbers again.
Let's recalculate the input impedance: without the 5M6, we've got 300 times (re + 2k2). re is still 45 ohms as gm hasn't changed. So that gives us 675k. Quite respectable.
What about the Miller effect on the the resistor? Well, assuming the voltage gain is x8.2, then it'll look like 610k. So...
(04-03-2017, 10:40 AM)ppppenguin Wrote: However since the input impedance is largely defined by beta x Re, and badly defined at that, the miller effect isn't going to make much difference.
Miller effect is more than halving the input impedance to 320k, so I'd say that's quite an affect. Nothing like what I erroneously suggested initially, but plenty. On paper, at least. The 2n2 cap changes all that. More on that story later...
On breadboard, fed from a 50 ohm source, and with no load beyond the 1M of the 'scope, the built circuit has a voltage gain of 7.64. As expected, it's not as high as the 8.2 suggested by the simple RL/RE approximation.
Next, I measured Zin using the added resistance method, using an old RS decade box in series with the input, and adjusting for half the output amplitude. I got 310k. Not bad!
For fun, I decided to eliminate Miller effect. To do that, I simply split the 5M6 in half, and grounded the mid-point via a capacitor. This obviously maintains the DC conditions, and the AC gain didn't change because of the relatively low open-loop gain.
Now Zin is 500k. Not as high as 675k, but to be honest, I wasn't surprised at that result, given that we're using simplified models and grotty breadboards. Also note that the 5M6 is being "Millered" slightly less in reality (because of the reduced gain) to 650k. But happily these "practical realities" converge nicely to give an overall input impedance that is within 10k (3%) of the prediction. That's pretty good, really...
(04-03-2017, 05:34 PM)Hartley118 Wrote: However, on a second look, it seems to me to be central to the whole basis of the circuit because Ca1 at 2200 pf is probably of similar order of capacitance to the crystal pickup, and what's more will be 'amplified' in value by the gain of the stage (Miller effect). Not sure what Ra2 is for because its resistance value only becomes significant compared with the reactance of Ca1 in the tens of kHz. Perhaps it went unstable without it! So forget its presence for a moment and what do we have? - an integrator.
Yes, I initially saw 2k2 and 2n2 and thought "33kHz" and ignored it. But 5M6 and 2n2 screams much louder! We can't ignore it after all.
I added the 2k2 and 2n2 cap. I've put the single 5M6 back in, so the circuit looks exactly like the original.
Fed from 50 ohms, nothing much changes at 1kHz. The -3dB point of the circuit has reduced (not surprisingly!) from ~70kHz to 4.3kHz.
But obviously the cartridge won't have a 50 ohm source impedance. That isn't specified for this model. So all we can sensibly do is plot Zin against frequency:
Code:
16Hz - 290k
31Hz - 247k
62Hz - 175k
125Hz - 102k
250Hz - 54.8k
500Hz - 27.5k
1kHz - 13.8k
2kHz - 6.9k
4kHz - 4k
8kHz - 2.88k
16kHz - 1.63kQuite a range!
For the record (groan!), I did check Zin vs frequency without the 2n2/2k2 network, and found that it falls to half of 310k at about 20kHz. Much less dramatic.
So for a given source impedance, the frequency response will change considerably as a result of this.
What does the 2k2 resistor do? I found that shorting it added overshoot to a square wave response. That's good enough for me - I didn't explore further.
So there we are. An apparently simple circuit that hides a wealth of intrigue within its many layers. And like the proverbial onion, the more you attempt to peel them back, the more you want to cry!
So when poor Mike innocently asked the simple question about input impedance, who knew it would be this involved?
Hope this is of interest!
Mark
