Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[Need help] breadboard of pre-amp circuit with BF245A is not working

Status
Not open for further replies.

nvha

Junior Member level 2
Joined
Oct 26, 2005
Messages
21
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,451
We are trying to made an pre-amp circuit for microphone using BF245A. In simulation it worked well. But with breadboard we got problem, the output is absolute silent.

Here is our circuit


Our data: as measured, Vg=0V; Vs=~0,4V; Vd=~8,5V. With "Soundcard" oscilloscope could obverse the electret mic give ~10mV before C1.

Any suggest for this?! Is the BF245 died? We changed other BF245 but result is the same. Or it was bad biased? Here is Vg - Vs = -0.4V.
The electret need to bias?
Thanks.
 
Last edited:

esp1

Advanced Member level 1
Joined
Sep 4, 2007
Messages
401
Helped
139
Reputation
278
Reaction score
138
Trophy points
1,323
Location
England
Activity points
3,369
hi,
The electret microphone requires a biasing/operating supply voltage.
Connect a resistor from the electret to the Vdd, say a 4K7 or 10K.
 

Attachments

  • Like
Reactions: nvha

    nvha

    points: 2
    Helpful Answer Positive Rating

nvha

Junior Member level 2
Joined
Oct 26, 2005
Messages
21
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,451
Eric, thanks.
We'll reply soon.
 

nvha

Junior Member level 2
Joined
Oct 26, 2005
Messages
21
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,451
Eric, it works very well. Thanks!
 

Audioguru

Advanced Member level 5
Joined
Jan 19, 2008
Messages
9,011
Helped
2,105
Reputation
4,204
Reaction score
1,919
Trophy points
1,393
Location
Toronto area of Canada
Activity points
56,611
Why do you use a FET? An electret mic already has a FET inside, that is what must be powered. Use a normal audio opamp as a preamp.
Edit: I guess you did not look at the datasheet for the VHF and UHF FET that is used at the input of high frequency radios. Yours is barely turned on.
Didn't you calculate the extremely low frequency that the 2uF input capacitor feeds to the 2M resistor? Earthquake frequencies down to 0.04Hz which is one cycle every 25 seconds. A capacitor value of only 0.004uF (4nF) will pass audio frequencies down to 20Hz.
 
Last edited:

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top