I need to power the Raspberry PI4 from my embedded system so i used a DC/DC regulator(https://www.sparkfun.com/datasheets/Components/LM7805.pdf) from a bipolar power supply composed of two 9v batteries the DC/DC regulator gives as output 5V and 1A but for the raspberry PI4 I need 5V 3A.Can anyone propose for me a solution to amplify the current or its ok to give the raspberry PI 1A as input current.I am new in this field.Thank you for the help.
The common 9 volt rectangular batteries cannot supply 3 Amps - they are designed for low current applications - I think even 100 mA is a bit much for them.
The LM7805 is a linear regulator and will waste over a third of the power you draw from the battery as heat.
Four AA cells and a DC-DC converter (switching power supply) may work, but 3 Amps is really too much for an AA cell - C or D cells would be better.
The RPi4 standard psu is 5V 3A. As mentioned linear regulators are not going to be very efficient in voltage conversion so you will be losing a lot of power and therefore duration the battery could have provided. You want to look at a switching based converter and a higher capacity battery technology. Take a look at https://www.adafruit.com/product/1385 coupled with a LiPo battery example https://hobbyking.com/en_us/turnigy-2200mah-3s-25c-lipo-pack.html?___store=en_us. A solution like this should give you around an hour or two of runtime. If this is not sufficient just scale up the battery.