Android Save text with transparent background

I am trying to render text by creating a canvas for a bitmap and the textview is drawn on the canvas and it is saved as a PNG image. I have 3 channels that do not have a 4th channel. Now how can I add alpha to the bitmap.

Bitmap testB;
strLogoImagePath = "storage/sdcard0/as.png";

testB = Bitmap.createBitmap(objTextView.getWidth(), objTextView.getHeight(),Bitmap.Config.ARGB_8888);

Canvas c = new Canvas(testB);

objTextView.layout(0, 0, objTextView.getWidth(), objTextView.getHeight());

objTextView.draw(c);


objTextView.setLayoutParams(rlLayoutParamsCanvas); 
FileOutputStream stream = new FileOutputStream(strLogoImagePath); 
testB.compress(CompressFormat.PNG, 100, stream);
testB.recycle();
stream.close();