Hi,
I have received a copy of Audio IC Op-amp applications (Walter G Jung) 1st edition , and I have been reading it all day. I feel it's a very well written book. FET inputs are recommended to overcome input bias limitations on slew rate.
Why is it most power amplifiers designs have bipolar inputs ? (often BC547 or similar)
I'm off to watch Brazil
I have received a copy of Audio IC Op-amp applications (Walter G Jung) 1st edition , and I have been reading it all day. I feel it's a very well written book. FET inputs are recommended to overcome input bias limitations on slew rate.
Why is it most power amplifiers designs have bipolar inputs ? (often BC547 or similar)
I'm off to watch Brazil
For an op-amp input impedance of higher than 10 Mohm may be useful at some applications, especially with precise offset or high source impedance. For audio power amp 20kOhm is more than enough because of how signal sources are designed.
Moreover JFETs are not very happy with high Vds voltages, like 30-50V, so for higher power amplifiers they need to be cascoded.
Moreover JFETs are not very happy with high Vds voltages, like 30-50V, so for higher power amplifiers they need to be cascoded.
...and the transconductance of JFETs is not determined by their drain current. So parts need to be matched for both Vgs offset and gm. Very expensive for manufacturing.
...and the gm of JFETs is rather low compared to BJTs for typical LTP currents in power amps. An LTP with 1mA tail current will enjoy 40mS bipolar gm but typically less than 10mS JFET. This lower gm may necessitate an extra gain stage in some designs.
...and a well designed amp is not necessarily slew rate limited by the LTP transistor types.
...and the gm of JFETs is rather low compared to BJTs for typical LTP currents in power amps. An LTP with 1mA tail current will enjoy 40mS bipolar gm but typically less than 10mS JFET. This lower gm may necessitate an extra gain stage in some designs.
...and a well designed amp is not necessarily slew rate limited by the LTP transistor types.
Hi,
Borbely uses FETs exclusively in his amps.
Have a look at Borbely's site and see the complexity he has incorporated into his front end.
Presumably, as the years have gone on, he has striven for more quality and this has progressively increased the number of front end devices he uses. Compare current to 1980 or so.
Borbely uses FETs exclusively in his amps.
Have a look at Borbely's site and see the complexity he has incorporated into his front end.
Presumably, as the years have gone on, he has striven for more quality and this has progressively increased the number of front end devices he uses. Compare current to 1980 or so.
Hi,
So it's possible but complex, and expensive!
I guess that leaves FET inputs for low level circuits (op amps, preamps,etc). I have looked at the AD823 http://www.analog.com/UploadedFiles/Data_Sheets/560004121AD823_a.pdf that has a jfet front end.
OP275:-
"This new front-end design combines both bipolar
and JFET transistors to attain amplifiers with the accuracy and
low noise performance of bipolar transistors, and the speed and
sound quality of JFETs." (OP275 http://www.analog.com/UploadedFiles/Data_Sheets/30371317475156OP275_c.pdf )
OP275 is on this site :http://www.dself.dsl.pipex.com/ampins/webbop/op275.htm
Is this just marketing (re-inforcing their market position) ?
How does a FET sound ?
So it's possible but complex, and expensive!
I guess that leaves FET inputs for low level circuits (op amps, preamps,etc). I have looked at the AD823 http://www.analog.com/UploadedFiles/Data_Sheets/560004121AD823_a.pdf that has a jfet front end.
OP275:-
"This new front-end design combines both bipolar
and JFET transistors to attain amplifiers with the accuracy and
low noise performance of bipolar transistors, and the speed and
sound quality of JFETs." (OP275 http://www.analog.com/UploadedFiles/Data_Sheets/30371317475156OP275_c.pdf )
OP275 is on this site :http://www.dself.dsl.pipex.com/ampins/webbop/op275.htm
Is this just marketing (re-inforcing their market position) ?
How does a FET sound ?
OPA604 datasheet - has another one of those 'FET notes' with a diagram.
http://www-s.ti.com/sc/ds/opa604.pdf (see page 12)
"Many audio experts believe that the sound quality of a
high performance FET op amp is superior to that of
bipolar op amps."
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1008,C1148,P1356,D4280
FET distortion increases with frequency but the distortion is easy on the ear ? (according to the audio experts)
http://www-s.ti.com/sc/ds/opa604.pdf (see page 12)
"Many audio experts believe that the sound quality of a
high performance FET op amp is superior to that of
bipolar op amps."
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1008,C1148,P1356,D4280
FET distortion increases with frequency but the distortion is easy on the ear ? (according to the audio experts)
AndrewT said:
Hi,
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=7206
I found this to be a good explanation.
I can see why few people / companies use them on the input stage. Matching the input fet's must be a real pain considering that they are relatively expensive , and used in cascaded inputs.
Here's another opinion
"FETs seem a poor idea for the input stage. The basic gm is so low compared with BJTs that there is little scope for linearisation by adding source resistors for local degeneration, so an FET input stage will be very non-linear compared with a BJTs degenerated down to the same transconductance: see 5.1.3."
From:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#7
"FETs seem a poor idea for the input stage. The basic gm is so low compared with BJTs that there is little scope for linearisation by adding source resistors for local degeneration, so an FET input stage will be very non-linear compared with a BJTs degenerated down to the same transconductance: see 5.1.3."
From:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#7
Actually, there is some rationale for this opinion, but it fails in practice when you consider the ORDER of the distortion components, input noise contribution, and bias current compromises. A fet stage can be quieter, have virtually no bias current, and have essentially the same magnitude of linearity as a bipolar input stage.
If you are willing to risk FM distortion, a bipolar pair can be acceptable, so long that it doesn't have any significant resistor degeneration . Test equipment input stages are one place where this might be acceptable, if the test equipment is designed for measuring AM distortion only. For hi Z inputs, however, fets are always better by far, for more reasons than I can describe here.
If you are willing to risk FM distortion, a bipolar pair can be acceptable, so long that it doesn't have any significant resistor degeneration . Test equipment input stages are one place where this might be acceptable, if the test equipment is designed for measuring AM distortion only. For hi Z inputs, however, fets are always better by far, for more reasons than I can describe here.
FM distortion is generated by the non-linear S curve that exists WHEN you want BOTH low noise and a bipolar transistor input. Degeneration resistor free bipolar input stages have a significant amount of FM distortion, unless the open loop bandwidth is very, very high, or the gain bandwidth is very, very high. You know: Very, very high implies qualities that higher than usually affordable by most people.
A good example of this is the AD797 op amp, or even the 5534 op amp and its relatives.
A good example of this is the AD797 op amp, or even the 5534 op amp and its relatives.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.