I am looking for help on wideband amplifier design for a battery operated device for ultrasonic testing. The ideal wanted performaces are:
- high (> 100kOhm) input impedance suitable for a piezo pick-up
- signal bandwidth from 50kHz to 250kHz
- gain 40 dB minimum (better if 60dB or 80dB)
- quiescent current < 50uA
- 3.3V single supply
Here is the whole story of my troubles, I think there could be enough to start a PhD thesiys... everybody's help would be appreciated!
--- OP-AMPs gain blocks ---
A quick overview of the available micropower monolithic op-amp finds out that the lower the quiescent current is, the slower the amplifer becomes.
For example a MCP6144 (0.6uA) has a GBW of 100kHz, too small to get any gain in the desired band.
1-A) If anyone can suggest op-amps that could fit both the GBW (>25MHz) and quiescent current budget (50uA total) it would be great!
--- discrete design ---
I found interesting paper in "Horowitz & Hill - The Art of the Electronics" pp. 950-952. It explains that when working with such small supply current all the amplification is obtained enlarging the voltage swings and in this case the parasitic capacitance of the transistors in the stages that follow the first one become the bandwidth limiting factor due to "Miller" effect.
The same book suggests a "Miller" effect workaround by using RF transistors with lower collector-base stray capacitance and even a ready-for-lab schematic (see picture attached) capable of selectable (!) 80dB amplification with a 20kHz bandwidth and only 10uA of supply current. Although the bandwidth is 1/10th of what desired and the supply voltage is 5V istead 3V I considered this a good starting point, hoping that, using better modern transistor and increasing the bias current I might be able to match my requirements.
I was not able to test the exact circuit of the picture since the 2N4957 pnp is obsolete and cannot be found anywhere. Anybody knows where 2N4957 can be found???
Then I tried to follow the circuit of the picture substituting the obsolete parts with modern low-capacitance microwave BJT, such as BFR505 and BFT93 and:
2-A) SPICE simulations seem not working, I suppose because the circuit is working far below the minimum collector current that is tabled in the models of the transitors. Is there any way to work around this problem?
2-B) even experimental test seem not working on the circuit of the schematic I derived from the original one (see pdf) on bench test it reported a maximum voltage gain x2 at 100 kHz, and tampering with the gain resistors/capacitrs had no success. What am I wrong with moving electrons??
2-C) from the same text the suggested schematic is mentioned as based on series-feedback pair gain stages but no detailed explanation of how to design this topology is given. The only extensive work I was able to found regarding series feedback pairs (M.S.Ghausi - "Optimum Design of the Shunt-Series Feedback Pair with a Maximally Flat magnitude Response" - IRE transactions on circuit theory) is really of little practical help... anybody would be able to help with design procedures for series-feedback pair topology???
If you have any alternative idea for such a terrific amplification task please advise!!
- high (> 100kOhm) input impedance suitable for a piezo pick-up
- signal bandwidth from 50kHz to 250kHz
- gain 40 dB minimum (better if 60dB or 80dB)
- quiescent current < 50uA
- 3.3V single supply
Here is the whole story of my troubles, I think there could be enough to start a PhD thesiys... everybody's help would be appreciated!
--- OP-AMPs gain blocks ---
A quick overview of the available micropower monolithic op-amp finds out that the lower the quiescent current is, the slower the amplifer becomes.
For example a MCP6144 (0.6uA) has a GBW of 100kHz, too small to get any gain in the desired band.
1-A) If anyone can suggest op-amps that could fit both the GBW (>25MHz) and quiescent current budget (50uA total) it would be great!
--- discrete design ---
I found interesting paper in "Horowitz & Hill - The Art of the Electronics" pp. 950-952. It explains that when working with such small supply current all the amplification is obtained enlarging the voltage swings and in this case the parasitic capacitance of the transistors in the stages that follow the first one become the bandwidth limiting factor due to "Miller" effect.
The same book suggests a "Miller" effect workaround by using RF transistors with lower collector-base stray capacitance and even a ready-for-lab schematic (see picture attached) capable of selectable (!) 80dB amplification with a 20kHz bandwidth and only 10uA of supply current. Although the bandwidth is 1/10th of what desired and the supply voltage is 5V istead 3V I considered this a good starting point, hoping that, using better modern transistor and increasing the bias current I might be able to match my requirements.
I was not able to test the exact circuit of the picture since the 2N4957 pnp is obsolete and cannot be found anywhere. Anybody knows where 2N4957 can be found???
Then I tried to follow the circuit of the picture substituting the obsolete parts with modern low-capacitance microwave BJT, such as BFR505 and BFT93 and:
2-A) SPICE simulations seem not working, I suppose because the circuit is working far below the minimum collector current that is tabled in the models of the transitors. Is there any way to work around this problem?
2-B) even experimental test seem not working on the circuit of the schematic I derived from the original one (see pdf) on bench test it reported a maximum voltage gain x2 at 100 kHz, and tampering with the gain resistors/capacitrs had no success. What am I wrong with moving electrons??
2-C) from the same text the suggested schematic is mentioned as based on series-feedback pair gain stages but no detailed explanation of how to design this topology is given. The only extensive work I was able to found regarding series feedback pairs (M.S.Ghausi - "Optimum Design of the Shunt-Series Feedback Pair with a Maximally Flat magnitude Response" - IRE transactions on circuit theory) is really of little practical help... anybody would be able to help with design procedures for series-feedback pair topology???
If you have any alternative idea for such a terrific amplification task please advise!!
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