Why Everybody use PNP instead of Diode in Bandgap?

[020170]

diode for bandgap

The picture is simple bandgap, voltage drop in Q5 and Q6 is Vbe1 and Vbe2 respectively.

Q5 and Q6 is replacement of diode. right?

Why everybody use PNP instead of diode to make bandgap reference?

thanks

 

[Fom]

You can use diodes if you sure in it's model.

 

[watersky]

The bipolar is used in bandgap, the reason is temperature. The formula of current is different between biploar and diode.

 

[yjkwon57]

Transistor rather than diode is used because of the reduction of bulk resistance.

Added after 3 minutes:

Another reason is for matching the dimension.

 

[Blackuni]

Hi,

despite the issues associated with PNP it is less noisy compared to its counter parts. Its implementation in IC's is simple compared to others.

Thanks

 

[rfsystem]

In CMOS a N+/NWELL/N+ structure ist used. The layout is a N+ diffusion, the emitter, surounded by either a Poly-Gate or LOCOS-Isolation and then a N+-Ring, the collector. The base is the NWELL. The substrate is an additional terminal. So it is a 4-terminal structure. Most connect them only as diode but there are other bandgap circuits which use the lateral PNP more intelligent. Resulting in more circuit performance. One issue in the circuit above is the bandgap voltage spread because of the offset voltage of the NMOS. The other is the PSRR because of the NMOS again.

 

[ambreesh]

The VPNP TC is chracterised much better than that of a diode.
And never trust the foundry data on TC' etc. better to have test structures and get your own data.