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Please take a look at my radio design

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Electronman

New Member
Hello guys,

I decided to design an AM radio with an Op-amp; it works with a 9V battery.
Ok here it is, Please put your comments if you think I can improve this circuit (I am learning Op-Amps yet).

I have a question too, I have used a coil and a variable capacitor out of a one channel Am radio for this circuit, But it just receives our local station (the signal becomes weaker when I turn the variable capacitor but I am not able to get another stations). What's the reason?
Does it comeback to the nature of my design or…??

Thanks.

P.s. does everybody know how many volts (I do not know if I am allowed to ask how many currents though) I should expect out of the coil part?
 

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Hero999

Banned
The performance won't be great as it's a simple tuned radio receiver.

For greater selectivity you need a to build a superhetrodyne receiver.

You'll need a resistor in-between C1 and D1.

Biasing the diode on all the time will help to increase the sensitivity.

Crystal Radio Circuits

The TL072 only works down to 7V which isn't ideal for running off a 9V battery because it won't completely discharge it.
 

Electronman

New Member
The performance won't be great as it's a simple tuned radio receiver.

For greater selectivity you need a to build a superhetrodyne receiver.

You'll need a resistor in-between C1 and D1.

Biasing the diode on all the time will help to increase the sensitivity.

Crystal Radio Circuits

The TL072 only works down to 7V which isn't ideal for running off a 9V battery because it won't completely discharge it.
Thanks,
I did not expect high performances, but receiving several stations not just one!

Why I need a resistor in series with D1?

How to bias the diode??

What do you mean by the last paragraph? Tl072 is able to work for much higher voltages???!

Nice link thanks for sugesting it.
 

Electronman

New Member
Yes, it's a crude crystal set with an opamp audio amplifer, but why are you wasting two opamps?, with the first doing nothing?.
Why are they called 'crystal' sets?
How can Improve it so that you do not call it a crude set?:eek:
I thought maybe the first stage op-amp will help to increase the very weak signal coming from the coil.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Why are they called 'crystal' sets?
Because pre-semiconductors (and pre-valve) you made crystal sets, using a gelena crystal and a 'cats whisker' to make a rectifier.

How can Improve it so that you do not call it a crude set?:eek:
More selectivity, more sensitivity - a single transistor reflex circuit would out perform this one easily. A crystal set, even with an opamp audio amplifier, is about the crudest (and poorest performing) you can make.

I thought maybe the first stage op-amp will help to increase the very weak signal coming from the coil.
It's giving zero gain, what could it improve?.
 

Hero999

Banned
Thanks,
I did not expect high performances, but receiving several stations not just one!

Why I need a resistor in series with D1?
How does C1 get discharged?

The diode can only charge the capacitor, i.e. it can push but it can't pull.

Connect a high value resistor from where C1 and D1 meet to 0V.

How to bias the diode??
See the site pointed to by the link in my previous post for an example.

What do you mean by the last paragraph? Tl072 is able to work for much higher voltages???!
The TL072 will stop working when the battery voltage drops below 7V, to get the maximum battery life, it's better to use an op-amp that will work down to 6V.
 

Electronman

New Member
Because pre-semiconductors (and pre-valve) you made crystal sets, using a gelena crystal and a 'cats whisker' to make a rectifier.
:confused:


QUOTE]More selectivity, more sensitivity - a single transistor reflex circuit would out perform this one easily. A crystal set, even with an opamp audio amplifier, is about the crudest (and poorest performing) you can make.
Yea I am agreed with you but just want to improve the selectivity performance. I will read the link and will reflex circuit (why it is called reflex circuit?)


It's giving zero gain, what could it improve?.
Please tell me If I am wrong, in several op-amp designs with virtual resistors ground I have seen a zero gain op-amp stage which its input is connected to the resistor divider and its output acts as virtual ground. So I thought maybe this stage acts like a Buffer ( gives more current at its output with the voltage value the same to its input I.e voltage gain of 1), Am I wrong?
 

Electronman

New Member
How does C1 get discharged?

The diode can only charge the capacitor, i.e. it can push but it can't pull.

Connect a high value resistor from where C1 and D1 meet to 0V.


See the site pointed to by the link in my previous post for an example.


The TL072 will stop working when the battery voltage drops below 7V, to get the maximum battery life, it's better to use an op-amp that will work down to 6V.
So I understood you wrongly.
I thought You were telling to put a resistor in series with C1.
 

Electronman

New Member
Can I replace that MPSA18 with a common transistor?
I am not surer if I am able to find it over here.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Can I replace that MPSA18 with a common transistor?
I am not surer if I am able to find it over here.
I've never heard of it either, but any silicon transistor should be fine, either general purpose or RF. Personally I'd use a BC107.

In case you're not aware, the transistor is been used TWICE - first as an RF amplifer, then as an audio amplifier as well - this is what gives it good performance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
In case you're not aware, the transistor is been used TWICE - first as an RF amplifer, then as an audio amplifier as well - this is what gives it good performance.[/QUOT]

:confused:

Did you look at my other post?
What?, about the buffer? - like I said before, it's doing nothing, and is in no way related to using an opamp to generate a split supply.
 

audioguru

Well-Known Member
Most Helpful Member
The Motorola MPSA18 transistor has been around for a long time. It has a very high current gain of 1100 typically and has spec'd low noise.
 

Electronman

New Member
What?, about the buffer? - like I said before, it's doing nothing, and is in no way related to using an opamp to generate a split supply.
So suppose if I need more curent for a circuit and have to use 2 resistor dividers, Can I use the Op-amp Buffer to do so? If no Why we use Op-am buffers? I have heard the answer to this question is 'impeance matching' But do not know if it has anything to do with giving more current (while making the volatge to be equal to the input)??

besides in accordance to my Op-amp radio, Can I use several turns of wires connected to the main ferrite somehow to improve the signal somehow (I do not want so much of quality).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
So suppose if I need more curent for a circuit and have to use 2 resistor dividers, Can I use the Op-amp Buffer to do so? If no Why we use Op-am buffers? I have heard the answer to this question is 'impeance matching' But do not know if it has anything to do with giving more current (while making the volatge to be equal to the input)??
Your second opamp is already giving exactly the same input impedance as the first one, which is why the first one is doing absolutely nothing.

besides in accordance to my Op-amp radio, Can I use several turns of wires connected to the main ferrite somehow to improve the signal somehow (I do not want so much of quality).
No, that simply alters the tuning.

You won't get any quality, it's AM :D
 

Bob Scott

New Member
Also, the TL074 op-amp has a GBP of 3 MHz, so maximum gain at 1.6 MHz is only 2X, or 6Db. Use a faster op-amp.
 

audioguru

Well-Known Member
Most Helpful Member
Also, the TL074 op-amp has a GBP of 3 MHz, so maximum gain at 1.6 MHz is only 2X, or 6Db. Use a faster op-amp.
The radio is just a simple crystal radio. The opamps are an audio amplifier for 100Hz to 3kHz.
 
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