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#1
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Sure, there are _op-amps_ that have cascode input stages. There are
bootstrapped cascode input stages used in feedback amplifiers that have extremely low input capacitance and very low distortion when driven from high source impedances. One of the FM tuner manufacturers long ago used a cascode input stage. With transistor amps, you can run the input transistor at low voltage and therefore low dissipation, and have the output transistor capable of large swings, and thus get good dynamic range. I don't know that there's any particular noise advantage, per se. But it does let you pick the best transistor for each part. Another variation with transistors is a "folded cascode" where the input transistor is one polarity (e.g. NPN) and the output one is the opposite polarity (PNP). Cheers, Tom oUsama (Yuri Blanarovich) wrote in message ... Back from good old tube days, cascoded triode RF preamps were good for high gain, stability and low noise. Anything out there in transistorised version, or there is there better stuff available? Looking mainly for preamps on HF to be used with low gain antennas, like small loops or beverages. Yuri, K3BU.us |
#2
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Thanks for pointers, I will have look at references mentioned.
Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU |
#3
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"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. |
#4
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"Ralph Mowery" ) writes:
"Yuri Blanarovich" wrote in message ... Thanks for pointers, I will have look at references mentioned. Another requirement would be good handling strong signals, like using 807 :-) I am trying to build the converter with strong signal handling capabilities, low noise and high gain. Next step is looking at the mixers and low noise synthesizers. Thanks a bunch! Yuri, K3BU You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Or, no rf stage at all. I can't remember what frequency this if for, if it was mentioned. An RF stage is only needed if there is need, like one needs front end selectivity to eliminate images or to keep the mixer from overloading, and the amplification is there to overcome the losses of the filter. Another need would be that a good low noise stage is needed because what comes later is too noisy. Michael VE2BVW |
#5
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Ralph wrote:
You are working at it wht wrong way. The three things you mentioned do not go together. What you want is somthting with strong signal handling and low noise. You use just enough gain to override the mixer noise. Then make up the gain at the IF stages after the filters. Not really. What you say is the general "wisdom". I want low noise, high (adjustable) gain preselector. Something with below 0.1 uV. There are situations when that is needed (small loops receiving antennas, Beverages on low bands, etc.) The gain should be adjusable so it is (band noise) just above the noise threshold of the mixer for particular antenna/band situation. At the same time have high dynamic range to handle strong signals S9+60dB without overload. Way back I modified Drake R4B by putting 6EH7 as preselector with separate RF gain control. It worked very well and it is still hard to beat. Yuri, www.K3BU.us |
#6
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#7
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By the way, did you check the "A High-Dynamic-Range MF/HF Receiver front end " by Jacob Makhinson N6NWP (QST February 1993 p. 23-28, with corrections in June 1993 p. 73) ? It contains push-pull preamplifier designs with feedback with nearly +33 dBm input IP3 and gain compression starting around S9+80 dB. Paul OH3LWR The mixer section is what is REALLY impressive about that front end. There was another article (also in the handbook) about a similar mixer that used a siliconix quad dmos fet that was even better. |
#8
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Paul OH3LWR:
While you definitely need the lowest noise figure and a high gain when using small magnetic loops, I very much doubt that you are going to need the strong signal handling capabilities for these antennas. Why not use a dedicated preamplifier constantly attached to these antennas, possibly with some protection circuitry to avoid damage when transmitting through your nearby transmitter antenna ? The idea is to have one preamp or RF stage for whole range of antennas 160 -10 or 6 m. No outboard boxes, preamps, but one good "in house" adjustable gain amp. No step or switchable attenuators either. Normal city mortals have high band noise levels, but when you go to the ocean front, use RX antennas or beam north on high bands, you need sensitivity and of course high dynamic range for the other end of the signals spectrum, like operating in Multi/multi environment. Thanks for N6NWP article info, I will look it up. Yuri, K3BU.us |
#9
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By the way, did you check the "A High-Dynamic-Range MF/HF Receiver front end " by Jacob Makhinson N6NWP (QST February 1993 p. 23-28, with corrections in June 1993 p. 73) ? It contains push-pull preamplifier designs with feedback with nearly +33 dBm input IP3 and gain compression starting around S9+80 dB. Paul OH3LWR The mixer section is what is REALLY impressive about that front end. There was another article (also in the handbook) about a similar mixer that used a siliconix quad dmos fet that was even better. |
#10
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Paul OH3LWR:
While you definitely need the lowest noise figure and a high gain when using small magnetic loops, I very much doubt that you are going to need the strong signal handling capabilities for these antennas. Why not use a dedicated preamplifier constantly attached to these antennas, possibly with some protection circuitry to avoid damage when transmitting through your nearby transmitter antenna ? The idea is to have one preamp or RF stage for whole range of antennas 160 -10 or 6 m. No outboard boxes, preamps, but one good "in house" adjustable gain amp. No step or switchable attenuators either. Normal city mortals have high band noise levels, but when you go to the ocean front, use RX antennas or beam north on high bands, you need sensitivity and of course high dynamic range for the other end of the signals spectrum, like operating in Multi/multi environment. Thanks for N6NWP article info, I will look it up. Yuri, K3BU.us |
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